I recommend staying away from magic for the first couple of months. I may seem like a good idea now, but trust me, you don't want to be "the magic kid." I did a lot of magic the first week or so of high school, and after a while, I absolutely hated being known as the magic guy, I wanted people to like me for me, and not magic. I haven't performed magic at school since, and plan never to do so again. Wouldn't you rather be known as "John, the guy who does magic" rather than "the guy who does magic?"
I may sound absolutely crazy right now, and honestly if the tables were turned, I would disregard your/my advice, but just trust me, you'll thank me later.
After you wait say, two months then you can start occasionally performing magic, maybe three or four times a week. Make sure you don't perform “challenging magic” (E.G.; complicated coin routines, the ambitious card routine etc.), nothing that seems like a puzzle. Teenagers will immediately see puzzling magic as a challenge, and try to find a solution instead of just enjoying the magic. That’s why visual magic is probably the best for school situations. Also make sure that the effects you perform aren’t generic effects that they’ve seen before, they’ll see you as less of a performer, and more of a guy who went down to the magic shop and wasted forty bucks. Finally, stay away from cards as much as possible, they seem more like sleight-of-hand, and less like magic. Plus, JoeStarnes is right, “…you want to be known as MagicMan not CardBoy.