I would like to see a 5% cumulative resistance per 10 levels to spells from your own school. I don't think that it's too unbalanced by level 70 to have a 35% resistance.
I proposed this idea as well, and there's still the dilemma of boost. Although monsters can resist their own school, they're vulnerable to their opposite school. For fairness, they may have to add boost. However, that's the least of the problems caused by this. How will resistance be instated? How will its addition impact equipment that already adds resistance to your wizard?
There is boost, prisms and yes there is the possibility that you could get somewhere in the neighborhood of 90% resistance to your own school's spells with school-only equipment, but at the cost of not being immune to anything else what with the way equipment is balanced. And what if you don't have any spells that are the monster's opposing school? That and the fact that I have yet to see a boss take a critical hit and dungeon monsters grandmaster+ can start out with more (sometimes many more) pips than you makes me suggest the school immunity. And, yes, hobi, I do check out the stats of my own equipment. With non-waterworks (which I haven't done yet) equipment I can have a 59% resistance to my own school but no resistance to anything else. Yes, yes, yes, hobi, after a certain level you are almost unstoppable and it takes a cheating monster or something like the 'Heart of the Heliphant' to keep you from soloing. It was just a suggestion, hobi, no need to get bowed up.