Which magic schools should I choose for the late game? Or should I go for a specific artifact?

I'll start with the end. Chaos magic isn't very good for mages (this magic is strong for Warlocks). Light can be replaced with mini-arts (initiative, speed, etc.). Dark magic is very powerful, allowing you to deal with very strong opponents! And Summoning magic is a classic for Mages! Summoning, Phantom, and Banish – that's what the Academy needs!