Awesome, thank you, very good advice. In that case, I think I'll go with an Elf Bishop instead and not focus on Piety.

Any advice on which schools I should focus on for my Bishop (or even whether I should try to advance all four equally)? This is one area I'm really unsure where to go. My other party members with spells would be Valk(Div), Sam(Wiz), Ran(Alc), and Gadgeteer (who'll have some gadgets with spell effects).

I would probably suggest going for Alchemy and Psionics first. Alchemy tends to be the hardest to raise if it's not ahead of your other skills because it shares so many spells, plus the best high level offensive spells are in this spellbook. Psionics is good because you don't have anyone else who can cast the spells, and I *love* the Psionic Blast spell.

It is still a good idea to raise Wizardry (Freeze Flesh/All) and Divinity (Rest All, Heal All (which can be replicated with items including 1 gadget), and eventually Restoration), but those skills don't need to be as high to easily increase when they're not your best skill.

Anyway, here is the situation with respect to leveling spellbook skills when they're lower than the others:

Wizardry: with 15 skill, you can use Enchanted Blade and Missile Shield (non-combat with practice timers); 30 gives you useful enemy-targeted spells (Fireball and Freeze Flesh)

Divinity: 0 skill gets you Bless (combat spell that doesn't backfire), 15 gets you Guardian Angel (ditto), and 30 gets you Rest All (cast in fake combat for practice), Armorplate (non-combat with practice timer), and Magic Screen (ditto).

Alchemy: 15 gets you Razor Cloak and Dracon Breath (combat no-backfire, but not that useful other than practice), 45 gets you Ring of Fire (combat no-backfire) as well as some useful combat spells.

Psionics: 15 gets you the useful combat spell known as Insanity, 30 gets you Hypnotic Lure (non-combat with practice timer).

When casting spells that don't backfire, or whose backfires are harmless (Holy Water and (in fake combat) Rest All), you can cast the spell at a higher power level for more skill experience. If the spell has a practice timer, you must wait a certain amount of time after casting it for the next cast to count, or you can take advantage of the fact that the timers aren't saved; just quicksave and quick-reload. (Note that quick-reload takes time on period computers; if your computer has decent RAM and an OS that does proper disc caching, this becomes much more practical.) Actually, that is not entirely accurate. It's true for healing and status recovery spells, but not true for others. In particular:

For combat spells (including things like Bless), you must be in combat against an enemy; fake combat doesn't count. (If the battle music is playing, you are in real combat.)

Long-duration spells (like Armorplate and Enchanted Blade) have practice timers; if you wait long enough, or if you save/reload, you can cast the spell and it will count as practice. (Note that the Portal spells also have practice timers, and that the timers are shared for the whole party.)

Stamina/Rest All are unusual: You can cast them outside of combat, but you don't get skill experience unless you cast them during combat. Fake combat does work here, however.

Summon Elemental: This combat skill does award skill experience when cast during fake combat. (Using this by a certain fountain is a good way to raise a Ranger's Alchemy so that she can learn Quicksand and Resurrect at level 18.)

Spells that are resisted still count. Spells that fizzle or backfire, I believe, actually give you *more* skill experience than those that are successfully cast. (This is unlike Dungeon Master, where failed spells give less experience, or Morrowind, where failed spells give none.)

I'd say Divinity is hands down the best school. That said many of the spells are available as potions and you got a Vakyrie and it's mainly buffs while you stated to prefer offensive magic. Mind Divinity is a trillion times better than all other schools on Hybrids. It's only drawback is the not-so-great realm usage (mainly divinity, other realms are used too late to help development). Never (!) pick Divinity damage spells - other schools are way better for damage.

Alchemy would go up nicely by mixing potions but you won't do that too much I understand? In any case strong offensive magic, not least DoTs (or rather Clouds). It's offense and healing really with a few (really good) tactical buffs. Decent realm usage. One may say it gets better the longer the game goes in terms of dealing damage compared to Wizardry.

Wizardry has nice strategical buffs but those are pretty easy to cast so there's no point training for that. It is the most straight out damage-in-your-face school, although the damage over time Alchemy offers is more damage overall. It uses all realms for damage, which is awesome as it allows to pump all your SP into damage if you want to, simultaneously raising most realms even early on. I'd definitely focus on this if you want offensive magic, not least because of it's realm usage.

Psionics has a few very good spells and a bunch of trash as I see it. A bit of utility, some damage, quite some debuffs... It is true that you'd miss out on some very nice spells if not developing it but I'd still be most likely to skip on this one if I had to chose one to ignore. In particular it is all about the Mental realm and does very little to develop the other realms. Other schools naturally see to it your realms can develop by offering good early spells in all realms. Maybe I don't do it justice and underrate it yet that's my honest impression.

So errrm... I'd level all of them :P
Not entirely sure it's wise w/o grinding but I'd try anyway. You got one Bishop - no point investing your skill points into any non-magical skills, right?
Thus I'd check which schools develop best naturally and potentially help the others with level-up points. E.g. if you end up using a lot of Wizardry in fights you may want to put more points into Alchemy instead of Wizardry to develop it. I'd favor schools over realms yet I'd also consider boosting seriously underdeveloped realms. Note that all schools but Wizardry have Heal - a spell only trains the currently highest school to have that spell on usage (thus the highest schools tend to rise quicker naturally). Also you want to get the skill thresholds for better spells (especially 15 / 30 for grade 2 / 3 spells) asap. The new spell may be just the one that sees to that school developing naturally.
Assuming you want the offensive magic I'd vaguely set priorities as Wizardry > Alchemy > Divinity > Psionics. Wiz & Alc are best for the realms, thus empowering overall magic better than the rest. Best offense as well. Divinity is pretty effective on a low skill but needs 15 (Guardian Angel - best spell in the game imho). Divinity 60 should be a target eventually and that's enough (45 Superman, 60 Heal All). Wiz needs 30 (Fireball), you can't have "enough" Wiz nor Alc.
A lot of it is but my taste so best do what feels right. If you have that awesome spell but can't cast it properly yet: put points in it's school or realm I guess, whatever "the development plan" was. Worst case - "omg misskilled everything"? Gotta grind a bit after all :P

A question: does somebody know the formula for Piety and gained SP? I guess it is a multiplier? If so: does it also multiply the extra SP per spell? If so It would be a lot more effective on Bishops than on specialist casters due to more spells, on top of a more effective realm usage. I do not know whether that is the case.

The effect seems to be smaller than you think. I just tested it on my level 18 Priest with 100 Piety and 25 fire, and an Ankh of Sanctity (+10 Piety) only gave 4 fire (with both fire spells) but 6 divine (with 70+ divine and, I believe, every divine spell except Death Wish). The effects seem to be more significant at lower levels, so I think only part of the equation gets multiplied by Piety. (Of note, without the Ankh, she has 120 fire SP and 271 divine SP.)

Being Hexed didn't affect the amount of SP gained from the Ankh.

For my Bishop (who has better magic skill but is of a lower level), the SP gains were 6-7 per realm.

As for the Bishop in general, I think he is only really good if you learn 3-4 spell schools and powertrain him at least a bit. I would even go that far and say taking a Bishop is sort of a concession if you don't have enough room in your party for 2 or 3 specialist casters. If I do have the room, specialist casters are always better and way easier to maintain.

For me the most common party setup is having 2 specialized casters, 1-2 specialists (Fighter, Bard, Gadgeteer, Rogue), and 2-3 hybrids/elites to cover the remaining 2 magic schools. Alchemy is the one spell school that is the most suited for a specialist, not only because it's the most powerful, but also because the respective hybrids are not really great with it. Ranger and Ninja usually both are great at ranged combat, so you usually won't use magic for that. Also, Alchemy really gets great at higher spell levels, which is not easy to cover by hybrids. Samurai and Monk both are melee combat specialists - here their magic can be used as a ranged weapon, and the important buffs of Divinity for a Lord/Valkyrie are all reasonably low level, which is good for hybrids as well.

Having said all that, I do think the Bishop is a great class, but not because of his power, but because of his versatility and most importantly the saved party slot(s). But if you love offensive magic and don't like powertraining, specialist casters are a better choice in my oppinion.

A few things to add:

Divinity: "Never (!) pick Divinity damage spells - other schools are way better for damage." Note that this does not apply to instant death spells. Instant Death and Death Wish are as good as death spells from any other spellbook; in fact, Instant Death is the first instant death attack available to you. Restoration is also nice; Heal All is, but less important thanks to the Regenerating Stone (and for parties with Bards, the Dulcimers of Mending).

Alchemy: Has Toxic Cloud, which is one of my favorite spells. Every round, each enemy affected has a chance of being hit with a status effect that may prevent it from acting. Battles aren't so dangerous when the enemies don't get to act. Also, note that there are no Mental spells in this realm (so combining it with Psionics isn't that bad of an idea).

Wizardry: Later on, the most useful spells of this realm are Freeze Flesh and Freeze All. Thing is, enemies that resist fire (like Rapax) are often weak against water, making these spells quite useful. (Also, Iceball isn't a bad spell if you would rather do a bit of damage.) Nuclear Blast is not as good as in past spells; it's frequently resisted, and Earthquake and Mind Flay are no weaker, unlike in Wizardry 6-7.)

Psionics: It has two really good offensive spells. Ego Whip is, for a while, the strongest available multi-target attack spell (you don't get anything stronger until you get 6th level spells), though it has the disadvantage of being group targeted. Psionic Blast is one of my favorite spells; it does the same damage as an Ego Whip of the same power level, can cause insanity, and most interestingly, is the only damage spell to be group targeted. (Insanity and Freeze Flesh have this targeting, but are pure status spells.) This allows you to hit enemies that you can't see, and if some enemies in the group are out of range, you can hit them as long as at least one enemy in the group is in range. (There's also Hypnotic Lure, but your Gadgeteer has that covered; on the other hand, it is a good practice spell (though it has a practice timer).)

In fact, barring the early buffs I think the Psionic spellbooks is somewhat better than the Mage book. There are some dubious things (Pandemonium, Prismic whatever, Mindread...), but most of the Mental spells are just really strong.

I've finally started adventuring now :)! Here is my party:

"Animal" (Dracon Fighter)
"Miss Piggy" (Dwarf Valkyrie)
"Janice" (Human Samurai)
"Fozzie" (Mook Ranger)
"Gonzo" (Mook Gadgeteer)
"Beaker" (Elf Bishop) *I decided to more-or-less try to advance all 4 schools for my Bishop equally, at least for now.

So far I've just been exploring the starting area, killing crabs, slimes, rats, and bats. The only fight which gave me any challenge so far were the 5 crabs on the way up the hill to the gazebo (I was 1st level at that time), but I managed to defeat them after a couple of reloads. Currently my fighter and gadgeteer just reached level 3, my others are level 2.

One thing I noticed so far, my spells seem to fizzle or backfire a LOT more frequently than in W6 or W7.

When you are about to cast a spell, you will notice that the rings around the power level bubbles are not all the same color. If the ring is green, casting the spell at that power level is guaranteed not to fizzle or backfire (barring situations like being hexed before you act (hence lowering your skills temporarily) or trying to use fire underwater). If the ring is red, a fizzle or backfire is likely.

To make more of the rings green, raise your skills. Realm skills make more of a difference here than spellbook skills, but both matter. Note, however, that your are limited by level; the highest level you can safely cast a spell at is equal to the number of levels you have been high enough in level to learn the spell. (Wizardry 6 and 7 had a similar rule, but it was a hard limit; W8 will let you attempt the spell beyond your means, but it is more likely to fail.)

Example: At level 4, with enough skill, you can safely cast a 1st level spell at power 4 and a 2nd level spell at power 2.

Have fun then!
Feel free to report some first impressions. I'd be intrigued to hear which things stand out to you ;)

I've finished exploring the Lower Monestery, and I'm just entered the Upper Monestery. All my characters are level 5, except my Bishop who's only about 500XP shy. I'm thinking I'll probably leave everyone at 5th Level until I get to Arnika.

So far I'm enjoying the game a lot, I really like how they've merged the real-time exploration with the phase-based combat, it plays out very well!

I've finished the Upper Monastery and begin my journey down Arnika Road. I killed a few Bandits down a side-trail and I now have enough XP to advance everyone to 6th level (except my Bishop), although I'm keeping everyone at level 5 for now.

You haven't reached the hard part of Arnika Road yet.

Btw, if you know what you're doing this crossroads area of Arnika Road is not that tough, it simply requires a bit of patience. Right south east of the crossroads (about opposite the Umpani house) there is a very deep crevice in the southern hills, where monsters don't see you and where you can camp savely. This can be used to just wait until no enemies are lurking around the crossroads and quickly sneak by. You can also lure enemies there to fight them one by one. If you feel like it you can even use it to powerlevel (if you intend to do this, you might want to save that for later). Either way, this hideout is a great place to become something like your Arnika Road "homebase".