From xiaojiangworld:

"I came back to take revenge on you." Obscene sounds echoing through an empty staff room. Fingers being pushed skilfully into a place where no one else has touched…" Minami, a target of bullying during his junior high school days, with his one and only friend Nozumu Kai. However, an event causes Minami to hurt his friend and a rift between the two friends is created, causing them to lose contact.
Ten years later, the two are reunited as teachers in the school of their past, and just when the distance between them begins to close, bullying reminiscent of their junior high school days begin to arise around Minami once again..

From Coolmic:

In the teachers’ staff room, he skillfully stuck his finger inside me as I let out a lewd voice. The revenge for what I did to him in middle school begins now.

Original Manga