Unstable Castle of Tokarev

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?