Hey i am a total noob at D&D so i am pondering one thing. Why does Yoshimo level up so quickly? My party consists of 6 people, and all of them are level 9-11 but Yoshimo is 13 and approaching 14

In AD&D (both 1e and 2e, though 2e is what's relevant here), rogues level up faster than other classes, so it's perfectly normal for a thief (or a bard, for that matter) to be a higher level than the rest of your party.

(Excludes druids at a certain level range, as their XP table is rather unusual.)

Experience Tables tells you the whole story. For example for 1.1 Million experience...

Bard and Thief become Lv 15
Druid Lv 13
Mage&Sorcerer Lv 12
Fighter Lv12
Ranger&Paladin Lv 11

because he nicks all the exp... I think the guy may be a thief ;)

That doesn't explain why a single class Druid levels up even faster, at least in the early part of BG2. (Note that this is only a temporary thing, and the Druid will fall behind while at level 12, when the Druid's XP requirements increase drastically for the next few levels, with level 15 not even being reachable if Throne of Bhaal is not installed.)

(If you had a single-class Druid in the party, they'd be level 12 right now I think.)

read it again mate
your answer is correct... my answer is a joke

But I felt like mentioning the quirky Druid XP table, and that was as good a post as any to reply to with that information.