There is no "quick fix" for this problem; it goes down too many layers. Firstly, while taking away access to broken BS is a good idea, this rule doesn't do that, it just slightly reduces the amount of broken BS a single mage can access. Secondly, I don't think we should be taking wizards down a peg, but rather boosting up non-casters to caster level awesomeness. Thirdly, no blanket fix will actually fix anything, and this is no different because a broken school of magic is not the same thing as a broken class/build; is the Illusion School T3 because it's rendered pointless by one of the most common immunities in the game, T2 because it has lots of combat and utility effects with a variety of uses, T1 because a good portion of the school is only limited by your imagination and creativity, or T-1 because Shadowcraft Mages who use Arcane Disciple (Luck) to abuse hyper-realistic Miracles can accomplish literally anything, despite technically only using one school of magic? Finally, and this is the biggest issue, its nerfing power does more to screw over non-optimizers than optimizers; veteran munchkins have lots of tricks around little fixes like this, but less obsessive players will be stuck being mediocre at best, and borderline monk-incapable at worst (if they have no idea what they're doing).