Houkago Hokenshitsu

From Go! Comi: Like most teenagers, Mashiro Ichijo has a secret. But Mashiro's secret is that he’s neither fully male nor female! So far, Mashiro's been able to live his life as a boy, but all this changes when he's informed of a new class he must take in order to graduate from his elite prep school. To pass, he must find the “Key”…and the only way to find it is to enter into a nightmare world where his body and soul are put at the mercy of the worst kind of enemies: his classmates!