The teacher isn't back, I guess he's out of the picture now.
At first I thought Chinatsu was going to bully the bully, but it turns out she's encouraging the victim to end her life rather than continue her hellish days in school. If it actually happens, Aiko's bullying of Enju could then get exposed and that would ruin her too.
Then Chinatsu would end up having destroyed two lives in one move.