[cheebacheeba]

Take it from a guy that was expelled over one violent incident after maintaining an almost perfect,...well...noit almost perfect, but an "ok" kinda record - If this shit doesn't get you expelled, which in all honesty, it could, even after a suspension...mind your temper.
Attacking the guy on school grounds was stupid, I don't care what the guy did...for that matter...the girl shouldn't take to heart what a fuckwit like this says, you should've told her this instead of acting upon it. Frankly, if she values his opinion on her physical condition, it's on her. You shouldn't have acted on it, in all honesty, and bringing a weapon into the mix is a whole 'nother shitload of potential trouble...however, sometimes you don't think when you're angry, I get that, and now you probably get it too - I trust this will be different in the future and maybe you'll think things over some.
For future reference, if you're planning on attacking someone, I'd suggest you do it outside school grounds, with minimal witnesses.

Think things through.
I hope this ends better for you than it did me.