Magic schools are very easy to level again because you still have your high level spells and your magica pool.
You can use casting cost reduction gear you didn't have the first time also.
Illusion and alteration are both easy, with Harmony and detect life in a highly populated area like Whiterun between the market and Gildergreen.
I haven't tried restoration, destruction, or conjuration.
They would still level a lot faster than 1h, 2h, archery, block or armor though I a pretty sure, except maybe destruction.