I know, this seems like a lot to ask, and it might not happen, but I just thought of it off the top of my head, and hey. why not listen anyway. Right?
I know that all the other schools have their opposites, and Balance is what keeps them all together, but what if there were a school that opposed balance itself?
Ladies and gentlemen: Havoc.
The symbol for it could almost look like a scale, except it would be turned upside down, hidden on a dark background icon and snapped somewhere at one of the ends.
The health of Havoc Wizards would probably be starting out the same as the Balance ones since they are kinda supposed to be polar opposites.
Balance is supposed to keep the order of the spells, so naturally Havoc is supposed to ruin it. I would picture letting Havoc have the spells that are always dispelling the other schools' charms so that they are at less of an advantage.