The manual lists the available Spell Focus options as Enchantment, Evocation, Necromancy, Transmutation. But this leaves out a number of the schools. Why is this?

Conjuration has some great crowd control spells and I need to get the DCs up as high as possible to keep sorcerors and priests at bay.

The omission of Divination and Abjuration make sense since there are no such spells that actually call for a saving throw in IWD2 (and very few even in PnP). Illusion has relatively few spells in IWD2 that afford saves so it's at least somewhat understandable. Conjuration is the big head-scratcher since some of the best spells in the game that require saving throws are conjuration school. I also ran a Conjuration specialist and was annoyed that it was missing.