Castaways

"Under our tutelage here at the Academy, you will receive a thorough instruction in occult history and practice, and learn to traffic with certain clandestine powers. You will face many ordeals, but you will emerge stranger than you have ever been. I'll change your life forever... if you'll let me."
Trapped in a dead-end town and a dead-end life, Amy Nettle dreams of escaping her abusive father and starting over, somewhere far away. The arrival of a black envelope heralds just that, in a way she never could have imagined. Whisked away to the Saunders Academy, a Gothic manse in the heart of an eternal storm-tossed ocean, Amy is one of dozens of teenagers plucked from dozens of parallel Earths and selected for an education in witchcraft. It seems too good to be true... except.
Except no one will tell them why they were chosen, or what happens after graduation. Or why the dormitories on the fourth floor are completely empty. There are tentacled leviathans and carnivorous mermaids in the water, a saboteur stalking the halls, and danger lurks around every corner. Worst of all, failure means the Arch of Resignation: a one-way trip back to where you came from, without your memories or your magic.
Falling in with a crew of misfits, Amy realizes they're all in the same boat: with nothing but ruin waiting back home, failure is not an option. Then there's Vail, an enigmatic tomboy who makes her heart flutter. For the first time in her life, Amy has something to lose.
Amy and her new friends will have to untangle the dark secrets at the heart of the Saunders Academy and master its mysteries, because there are only two choices left: graduate, or die.