Generally to encourage the villagers to turn against the partisans... i mean troublemaking students.

To find the skibidiest once and for all

The teacher is hoping the rest of the class will socially punish the disrupting student in some way. They are doing this because teacher have no means of punishing students other than this and scolding.

It was probably more than just one kid being rowdy and that was just the last straw. Could be just a shitty teacher though.

Performing your part in a struggle session is not a punishment