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?
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.
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 : https://en.uesp.net/wiki/Skyrim:Destruction_Ritual_Spell#Bugs