Kurayami no Hate de Kimi wo Matsu (2009)

During the middle of an outdoor hiking event at your school you suddenly get sick.
It was decided that you should go back to camp by bus along with the student council president Aoi Mio, your childhood friend Sakuraba Katsumi, members of the tennis club Honami Yousuke and Akiyama Tomo, a student from another school Kazano Tarou, your teacher Kousaka Takahiko, and the trainee teacher Mikomoto Naoki.

However along the way you met with an accident.
When you came to, you and everyone else on the bus were trapped in an unknown abandoned school.

There you found a letter from a person calling themself "Death Mask". It read "If you do not escape from this school according to my orders you will meet with death."