Tokarev no Ayaui Shiro

A close friend of Ayaka, Hiroko, left high school and disappeared, explaining only that she had a significant reason. Years later, Ayaka finds a letter from Hiroko attached to a dead girl's body, requesting assistance. Determined to locate her, Ayaka travels to a remote all-girls high school that unexpectedly welcomes her as a student. The school appears to have unsettling intentions, and Ayaka must uncover its hidden truths to save Hiroko before it's too late.