As you may know, when a Bishop casts a spell, she normally only gains experience in the spellbook in which she has the most skill, right? Well, I found an exception to this rule.

Situation: Bishop has 78 Divinity, 75 Alchemy, and is Silenced. Bishop casts Heal Wounds. After casting it, Alchemy increases to 76.

Conclusion: If a Bishop casts a spell while silenced, the skill experience always goes to Alchemy, even if the spell belongs to a spellbook with which she has a higher skill.

Just another quirk of the game mechanics, and one that you can use to your (minor) advantage, if you have a Bishop whose Alchemy skill is low and needs improvement.

(Of course, be aware that only spells in the Alchemy spellbook can be cast while silenced.)

That's interesting. So if you want to increase Alchemy with the Knock Knock or Heal Wounds spell, but Alchemy is lower than Wizardry or Psionics/Divinity, just let another character cast Silence on your Bishop and train your Alchemy spellbook.

Unfortunately, the game engine does not allow you to cast offensive spells on party members, so that won't work. You need to get an enemy to silence you, which is not so easy, especially since silence is temporary and wears off quickly. (Or you get an enemy to cast Eye for an Eye, and then cast Silence on that enemy, which is more practical since the game *does* allow you to cast beneficial single-target spells like Eye for an Eye on enemies.)

Ok, that works, but it doesn't sound like it's worth the hassle

78 Div & 75 Alc is a sure sign of either non optimized development or a looong battle that raised Div over Alc.
Time to mix some potions.

Actually, I kept her Alchemy low through much of the game because my party also has an Alchemist.

In any case, I find Restoration to be the most useful of the 7th level spells, and now that my Bishop has 81 Divinity and 97 Divine Magic, she can learn it at the next level up (she's level 17, which is high enough to uncurse the Staff of Doom without using a spell. Does level 16 work? (level 15 doesn't.)). Thing is, unlike every other 7th level spell, Restoration works well regardless of the enemies' resistances.

Also, with spells like Toxic Cloud and Quicksand around (and known by my Bishop), mixing potions to get those last few skill points shouldn't be necessary (although casting Quicksand at unsafe levels could lead to multiple character deaths if I get unlucky).