Lonely Castle in the Mirror (Manga) Vol. 2 - Mizuki Tsujimura - Bog

Seven junior high students are granted access to a mysterious castle. The only thing connecting the kids is that none of them go to school during the day. Kokoro sees the castle as the only place where she truly belongs, but not everyone feels that way. Ureshino announces one day that he's going back to school, but as summer vacation comes to an end, Ureshino returns with mysterious injuries. As it becomes clear that everyone has their own personal secrets, Ookami-sama announces a new rule that will change everything.