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

I would choose the following schools: Summoning, light/dark (depending on the opponent)... Chaos isn't really a core school for the Academy, is it?

No. But neither is darkness. However, you can find spells in the guild, there's a library. But in the late game, I'm leaning more towards Summoning with darkness.