Firstly, don't go saying that "normal" high schoolers drink. Many of them don't, and are still considered "normal." In fact, I've never placed all that much stock in the definition of normal to begin with. It sounds far too boring for me. If not drinking makes you unique, then I feel as though that's even more of a reason to remain sober.
As cliche as it sounds, you're experiencing peer pressure. If you're smart enough not to drink, then chances are, you're smart enough to already know that. Don't give in to it... high school goes by fast enough. Don't make it go faster by experiencing it through a drunken blur.
I suppose, though, if the people you hang out with are that important to you, you could try to beat them at their own game. I'm guessing that most high schoolers will get the booze they can afford to have their older siblings/cousins/friends buy for them, which is usually the cheap stuff. If you're at a party and someone asks you why you're not drinking, say something to the effect of, "Oh no, I don't drink beer (or whatever). I only drink the highest quality Kentucky bourbon, aged for at least four years." You'll seem classy, slightly baddass, or slightly snobby, but I suppose that depends on what type of people you choose to spend your time with.
Or you could just say "No, I prefer my liver the way it is." You choose.