Regardless of whether the knife was real or plastic, it doesn't matter. He still set her hair on fire, and it still could have had tragic consequences. Excusing his behavior because the knife wasn't "real," or rather, implying that the fact that it was plastic is a mitigating factor, is an example of why kids who show violent tendencies end up being adults who follow through on violent tendencies. They aren't made to take responsibility for their actions, so they learn that they can get away with it. Then they get more and more bold and more and more violent, and eventually they end up killing someone because no one ever stopped them before it became lethal. THEN their lawyer says, "but it's not his fault....it's the school's/parent's fault for not stopping him earlier, so...."
I'd've made him wish he'd never seen that girl if he'd been my son. He'd have made a public apology to her, and he wouldn't have seen daylight for months.
There is absolutely NO excuse for his actions. None. Nip the behavior in the bud before it gets a chance to get out of hand, I say. Hell, those "zero-tolerance" schools out there have EXPELLED kids for bringing plastic knives to school.
You can still poke out an eye, you know...
Sidhe