I understand that when a magical skill such as Destruction reaches 90 you are supposed to be able to receive a special radiant quest from the master that skill (Faralda, in the case of Destruction) where the rewards are more powerful spells in the school. But though I have achieved a skill level of 90 or more in several different schools of magic, I canno get any of these quests to start. For instance, my illusion skill is 98 and the master of Illusion is Drevis Neloran, but no matter what I do or where he is I still can't get him to offer me the quest. The same is true for Destruction/Faralda and and the other schools of magic. Is there some other requirement I don't know about, or is there a console command or some such I can use to get these quests started when my skill levels are 90 or more?
There is a slight bug in this quest which causes it not to start until the Dragonborn reaches level 100 instead of the correct level of 90.
This is fixed in version 1.2.6 of the Unofficial Skyrim Patch:
Two of the master spell ritual quests (Illusion, Destruction - MGRitual02, MGRitual01) have conditions set on their initial dialogue branches that should not be there because they block access to the quests unless the skill level is already at or above 100. This is inconsistent with the remaining ritual quests. (Bug #8605)
You can use this console command to start the quest:
setstage MGRitual01 10
Also if you run into similar problems with a quest not starting you can look at the bottom of every quest page on UESP for a command to manually start the quest.