The Scholomance,

For centuries, there has been a legend of a hidden school where magic is taught by the demons who dwell there to anyone who seeks them out, but they ask a terrible price: Anyone who reaches the door of the Scholomance may enter, but the Devil takes every tenth student who tries to leave. A hidden school. Demonic masters. An inescapable fate for one out of every ten graduates. But Connie would do anything to have the magic her best friend was born with. And Mara would do anything to get Connie back.