Sacred scars - Kathleen Duey

Having been driven out of their home by fire, Sadima, Franklin and Somiss escape and seek refuge in the secret cave hidden in the rocks. Somiss, now exiled and desperate, continues to hoard the magic he is recovering from the ancient documents, while Sadima and Franklin struggle to contain his egomaniacal ambitions.
In Haph and Gerrard's world, forced to continually endure the painful ordeals used to 'teach' magic, the boys come to an uneasy and fragile truce as they vow to work together to stop the evils of the academy. But their tenuous pact falters as they plot to destroy Somiss and the dark academy and set the magic free…