How about, instead of a random school, it procs with the same spell type as the spell that procs it? Then it won't just be some extra damage on the side, but something that synergizes with the build of the caster. Frost can have more chances of proccing FoF, Fire can crit with it and proc Hot Streak, Arcane can...do more damage with Arcane spells. And it doesn't have to be just the 3 mage elements. Do Shadow, Nature, and Holy too. Just so it feels worth using and adds some flavor to it. It doesn't even have to be just the 2pc (tho I prefer it is). You could make so the 8pc has an additional effect of making it match the spell's type (as I explained).
Without numbers, I can't really gauge how much damage this will do. But if it synchronizes with the build from the beginning, it feels like using it may be better just for that reason.