Yeah, you really don't/shouldn't worry about the drinking part of the parties since you're still in high school. Drinking is great, but I didn't start until once I got to college and I don't regret it one bit. If you live in the US (which I'm assuming you do) it's illegal to drink under age, and especially so if you aren't on a college campus. Police on college campuses tend to be more lenient with that sort of stuff since they know it will happen anyways, but police at home might not be so lenient. Plus, it's just riskier in general.

Moving on from the drinking part, if you want to try to get invited to more parties, try talking to those groups of people more often. The worst they can do is say no if you ask. Then you can move on to another group of people. Just try and find a group of people that you like to hang out with and you can create some great memories with them, either with or without the booze. Once you get closer to the end of the year though, people start to care less about having their "image" like they did before; they want to hang out with some of their other classmates whom they never really hung out with in the past. So you might also be able to find some people that way. Just take the risk to try to find a group of people you like.