Bukimi de Soboku na Kakowashita Kimi to Boku no Kowareta Sekai

The fourth instalment of the Sekai Series. For him, everything is a game. Seven mysterious murders strike a prestigious girls' high school! A mysterious and inexplicable murder takes place at the supposedly peaceful Senju Girls' School. What's more, the person who works there is the ethics teacher, Kushinaka Choushi. Fourteen years have passed since the incident involving Byouinzaka Meiro. The search for the culprit in this game of detective starts again. Who is the culprit? (Source: Kodansha, translated)