Kurayami no Hate de Kimi wo Matsu

A school hiking trip ends abruptly when the player falls ill and boards a return bus with a small group: student council president Aoi Mio, childhood friend Sakuraba Katsumi, tennis club members Honami and Akiyama, a student from another school, the homeroom teacher, and a student teacher. The bus crashes. When everyone regains consciousness, they are trapped in an unknown abandoned school and find a letter from someone calling themselves "Death Mask" — warning that unless they escape according to the letter's instructions, they will die.