I'm not very good at this - how about he show them a video of Hiroshima being bombed and promising it will happen to their city if they ever threaten his child again? He doesn't have to tell them he couldn't do it.
Well he sent a kid into a dangerous forest (probably so he could be alone with the girls.) I'd say he is a first-class jerk. I doubt he is the sword hero. Maybe the kid really is.
What does that vice-principal know? Is she the teacher from his past? No, I doubt that. Still she knows more than she should about him. Is a plot actually going to develop?