Reminds me of "The Prestige".
I say do Flow and let the water pour on his head, then amp up an Electric Touch and fry him, then suck all the iron out from his body with the M5 (think Magneto), or upgrade your M5 to an M16 and fill his body with bullet holes, then proceed to paint the town red... with his blood.
In all seriousness though, don't do any of those, or anything else that can merit you a punch in the face. They're out of the question. You'd know what those are anyway. These are natural in school settings, and not always related to magic mind you.
As tempting as showing him off might sound, this isn't the best way to handle the situation. Honestly, to have a mutual relationship with people like those sounds a bit too difficult to achieve at times, and I know for a fact that resorting to telling your parents or the authority about your problem can take it's toll on your pride. It's how it works these days, or around here at least.
Try to assert yourself passively, if that makes sense. He wants to be the more superior magician and he means to get that through intimidation. If those don't happen, what else can he do? If you, in fact, are less capable than he is, what have you to lose? You status alone as a freshman is already a factor. People would soon realize that he's just insecure and that he's no better than what he wants everyone to think.
If it does come to a "duel", chances are he's aware of the material you'll do. With or without competition, your best bet would be to perform your best and to entertain the audience, not to show off the other. That's what you're supposed to be doing in the first place and that's what the people expect out of you.