The Wizard Heir by Cinda Williams Chima

16 year old Sep has spent the last three years getting kicked out of his private schools. It's not his attitude or his behavior, but his very magical abilities that keep accidentally exploding that is the issue. Seph is a wizard, orphaned and untrained, and his powers are getting out of control. After causing another fire, a secluded boys school takes him in. At first, everything is great, until Seph realizes that his headmaster uses the students powers for his own gain. Seph is now in the middle of a war and is not sure he will be able to survive.