Kigagiga

Keiji Sakuragi is a second year student in a prestigious school Saji Academy.
While he lives in busy days as the head of the school council, he is suddenly
confined in a locked room that he has never seen. In this room, his classmate,
school teacher, younger student and a "man of the type that he hardly interacts with".
A man tells Kieiji that... "Everyone in this room are not human."
When the relationship among five is distorted by this extreme circumstance,
some abnormal changes arise from their bodies...

[From DLsite English]