I'll Bully You

Minami was a target of bullying during his junior high school days, and Kai was his one and only friend. However, a certain occurrence causes them to lose contact.
 Ten years later, the two are reunited as teachers in their old school, but just when the distance between them begins to close, bullying reminiscent of their junior high school days begin to arise around Minami once again...

(Source: Seiunsha, translated)