Level up at least three creatures of the desired school, with a cooldown of no more than 3 turns.
Find neutral Dungeon creatures on the map and, in battles against them, leave only their brethren. The creatures won't attack each other, and you can keep casting spells and gain +1 reputation point each round. It's tedious, but with enough patience, you can accumulate as many points as you want. I collected 20, but then I stopped because it became boring.
Win the battle with the hero's attacks, unless he's a Necropolis mage, of course.