Tokarev No Ayaui Shiro

ayaka has a dear friend named hiroko. hiroko suddenly vanishes from ayaka’s life, saying that she must save someone important to her. since that time, ayaka hasn’t heard anything from hiroko, until a girl’s corpse is washed up on shore, and she’s holding a letter from hiroko which reads: “come save me.” ayaka sets off to save her best friend, and ends up in a mysterious school where everything seems too good to be true. is it?