Kokubu-sensei isn't going to stop until Takagi is made to realize what she's done.
Sure, it'd be great to see 'punishments', but he wants to make a difference in everyone's lives, not just the victims. He's working to be a teacher for those led astray, as well, instead of just passing the buck to someone else to fix the problem.
Vengeance won't teach them a damn thing except to not get caught next time.