Firebreak

When Amy Nettle was whisked from her dead-end small town life to the Saunders Academy, a brooding Gothic fortress in the heart of an endless storm-tossed ocean, she was promised an education in witchcraft.
She got more than she bargained for. Beyond the brutal lessons that pushed her to her mental and physical limits, there were leviathans and killer mermaids in the water, lethal magic in the air, and an invader determined to make sure none of the new students survived long enough to see their graduation day.
Then there was Vail, a freckled tomboy with a knack for pyromancy and a lasso wrapped around Amy's heart. Together with their fellow castaways, a band of misfits from a handful of parallel realities, they exposed the plot against the school and sent its architects on the run. The enemy is still out there, though, in the wilds of Firebreak Island, and the Academy is battening down the hatches and going on a manhunt.
There's danger in the woods, but even more inside the fortress walls. When warring rivals drive a wedge between Amy and Vail, their loyalty is put to the test. Worse, there's no denying the facts: one of the students is a traitor, trying to destroy the Academy from within. Their second year of occult education has just begun, and if the castaways can't uncover the truth in time, there won't be a third.