[@Raizin] I think you mean 'schools' or D&D calls them 'domains'-different types of magic that can be mastered. Which I think is a great idea so were not all throwing conflicting ideas out there. Couple things you have to ask first:

 How prevalent is magic in the setting? Do average people have access to it or does it take years even centuries to learn spells? 
 What is the source of magic? A few common sources are divine, elemental or magic simply being another natural force in the universe.
 What are the drawbacks to using magic? What toll does it take on the mage? 
 
 All I can think of at the moment-but it should get people thinking about magic in the setting.