I was under the impression that you could do the Master quests once you reached 90 in that school (and I've already done Alteration at 90), but my Illusion skill is past 90 and I still don't have a dialogue option from the Illusion trainer at the College. Is Illusion different for some reason? Does it require 95 or 100, maybe? Do any of the other schools require a skill other than 90?

Thanks.

I just scanned through the UESP, a known bug is not buying a spell from him will cause him to never offer the Master Ritual quest.
I did not have any problems on my Mages, but my characters would always purchase a spell or two while recieving the "Cleansing the Fonts" quest.