Dark Hunter (Jou) Youma no Mori

Mariko Kayano is a student at Jigen High School, a remote academy for troubled youths, who was sent there after she started seeing visions. Two strangers, Ran Mashimo and Cher Mulligan, show up at the school in the guise of transfer students, and soon the dark secret of the school begins to make itself known: the school's founder and headmaster, Shogen Kuji, is in fact a pawn of interdimensional parasitic creatures bent on consuming and destroying humanity, and many people at the school have already been infected and consumed.