In Japanese folklore, Oni are evil creatures such as demons, ogres, and trolls. When the Oni (delinquents) at Onigashima High get unruly, Momo hires Otogi Bank to address the issue with the student council president.
From there, you should be
able to reach the main building
Ihr solltet von euch aus über die Brücke
im dritten Stock ins Hauptgebäude kommen.