ODAN

"Boulder" is an experimental new boarding school that is dug deep underground like a bunker. The school focuses on two ideas: strict discipline and anonymity. Each student has a unique mask that they must wear at all times, their real names are replaced with numbers and they aren't allowed to speak about their true identities. Breaking any of the rules leads to severe punishment. Even when teachers aren't around each class has an ostiarius, student who is authorized to punish their classmates for any violations. Despite everything students of class 1 still manage to find some happiness in their lives.

One day mysterious monsters attack the school. The only chance to survive is to escape from the school and for that the students must find a key to the front door. Unfortunately all teachers are gone and besides them only ostiariuses have keys and class 1's ostiarius disappeared just before the monsters appeared. Now the students must either find the key or figure some other way to escape.