Kimi no Nemureru Kaii no Kousha

Ren Nakazato, who has just entered high school, visits the old school building to experience the "Top Exploration Club," which is rumored to be studying the school's seven wonders. There, Ren meets three first-year students, male and female, who are also there to experience the club. When no one appears to be a member of the club, Ren and the others decide to disband, but they find themselves trapped in the old school building. The four were impatient and found a message in the classroom...?

(Source: Comic Natalie, translated)