Kuma is a teacher who ended up in a physical relationship with one of his students. They were not dating and only went all the way once but Kuma told him to ask him out again after the student became an adult.
Chiaki avoids his math teacher, Chiba, since he does not want to be tutored. One day he forgot his phone so goes back to school and sees his math teacher having sex with the PE teacher. Chiaki took a photo and decided to blackmail said teacher but to no avail.
Ijimete Hoshii no?