Well, you are 15, so i think your classmates probably know what makes you a boy by now...that's called maturity. When it comes for tights vs. sweats, tights work better in the long run, since they stretch further. I don't think they are that uncomfortable. If you want to get used to them, just wear them around the house a lot (even under your sweats!). Fears over your clothes shouldn't make you give up ballet....it's not like they are making you dance naked!
Remember, the tights that guys wear aren't the sheer kind that you see on girls. Our tights are opaque, and you can't see through them.[/i]