Construction

The level of your construction research determines which magic items you are allowed to craft. Some construction spells also allow you to make magic units. - Illwinter.

Needed if you want fancy items. its also anouther school with manly summoning spells as you construct minions. You may also pick this up for earth wizards for troop buffing.

Generalization

Standard

Fire
Air
Water
Earth Create mechanical warriors. also Great for buffing troops with Leigons of Steel, and Weapons of Sharpness.
Astral
Death
Nature