Create 4 sorcerers and assign each one a school, and name each one FIRE, WATER, AIR and EARTH.
Sell the 3 starting Flame Arrow/Fire Bolt books that WATER, AIR and EARTH sorcs started with because they are forbidden to cast spells not of their eponymous school, obviously. Nor can they cast the Torch Light spell they started with.
Should be fun.
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It's up to you if you decide they are allowed to use dark and/or light magic or not.
This challenge can also be tried in MM8, if you create a Necro as starting character and hack 2 of the earliest characters into Necromancers (that Fredrick priest guy and the Vampire lady. Can easily be done with a save game editor, BTW)