Why are people ignoring me, and what should I do about this?

There are quite a few things you can do about this. First off I want to say that I know it's hard being in high school. Wasn't my favorite time either. But it does sound like something is going on behind your back. I would suggest to just confront one of them privately and straightout ask them what's going on. If you confront in a group, like the ones you're talking about, they are more likely to say 'I don't know what you're talking about' or 'Nothing! We swear!'. Confront one and it seems like the truth comes pouring out. Then, you can take it from there. Don't keep sweating it out and don't fail your cles because of something stupid. Good grades follow you to colleges and jobs. Stupid people at school don't.