The Last Graduate by Naomi Novik 

At the Scholomance, El, Orion, and the other students are faced with their final year–and the looming spectre of graduation, a deadly ritual that leaves few students alive in its wake. El is determined that her chosen group will survive, but it is a prospect that is looking harder by the day as the savagery of the school ramps up. Until El realizes that sometimes winning the game means throwing out all the rules . . .