What are the best magic schools for a "glass cannon" priest with high int? The ones with the best damage.

It has been a very long time since I played Inquisitor for the last time, but there a few generally useful spells it would be wise to learn: Holy Ground & Divine Mercy from the Book of Divine Magic that will allow you to transport from dungeons to towns and back, Levitation from the Book of Magic of Miracles that will allow you to overcome gaps and lava/acid pools, Sixth Sense and Light from the Book of Inquisitorial Magic (the first one for free item identification, the second one to light your surroudings in dark areas), or Decay from the Book of Heretical Magic to easily open locked chests.

And looking at my final priest build, I can see I have Rain of Fire from the Book of Pagan Magic and Lightning from the Magic of True Faith in my action chain as my strongest offensive spells, as well as Purge from the Book of Magic of True Faith to get rid of various negative effects.

But if I am not mistaken I was changing my offensive spells and switching between schools throughout the game based on what spells I learnt from the scrolls and what spells I was actually able to cast given my stats - you cannot cast Lightning until you reach the Initiate level or Rain of Fire until you reach the Magister level in respective schools which means you have to use offensive spells from other schools until you level up enough (I believe I was using Open Wounds from the Book of Inquisitorial Magic for quite some time because that is a Novice level offensive spell.)

I am not sure if I helped you but that is all I can say about the subject. :-)