From MangaHelpers:

One Saturday, a group of students met at school to work on a project for the school festival. While taking a break from their work, they suddenly lost consciousness and awoke to a darkened school building. Every door and window seemed to be blocked solidly by a stone wall. What has happened? Is this the result of a classmate's curse? Did the school somehow sink? Have they been transported to another world? Whatever has put them in this situation, they're trapped together, and they have a limited supply of food... [tethysdust]