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

...isn't chaos kind of an off-specialty for the Academy?

There's one hero named Nazir; he starts with chaos and is very strong in the mid-game, dealing a lot of fire damage. It's a shame that he becomes predictable. :( But overall, he's a good hero; the fireball spell at level 1 is really cool!!! :cool:

Added 5 minutes later
You can also take chaos for other heroes because the Phoenix Feather, Ice Amulet, and Trident of Titans with Emerald Boots haven't been canceled out – and they are quite often found on the map – but the Ice Shield isn't very common, unless it's in an artifact shop. :)