The Siege (Guardians of Ga'Hoole, Book 4) by Kathryn Lasky
Fourth title in a mythic adventure series in which the heroes are owls!
In the owl kingdom, a war between good and evil is raging. On one side, Soren and the noble owls of the Great Ga'hoole tree. On the other, Soren's fearsome brother Kludd, who wears a terrifying metal mask to cover his battle scars. Driven by an all-consuming lust for power, Kludd leads his forces in a brutal attack on the Great Ga'Hoole Tree.
Meanwhile, Soren is asked to lead a mission back to the one place he never wanted to see again - St Aegolius Academy for Orphaned Owls. He and his crew will have to enter St Aggie's as spies, then leave without being noticed...or caught.
Soren escaped the rocky confines of St Aggie's once. If the Ga'Hoole tree is to be saved he must do it again...