This has been bugging me for a long time now. Whenever I meet someone (as in saying hi to someone I know) they act more restrained and withdrawn around me than they do others. For the most part, no one makes any effort to talk to me, and they never do, either. I even find some people glaring a hole through me and talking bad about me behind my back! I know that it's not normal. All I'm asking for is advice. What should I do? (I'm a freshman in high school, if it helps.)

Either case, you shouldn't bother with them. If they're rejecting you, they aren't a friend you'd want to keep. Be grateful in that sense. Of course, I could see why it would still upset you.
Try Checking out the way you talk and your facial expressions. It might be that simple, i had that problem because im always thinking about something else when im talking to others. People are sensitive and pick up on it.
Body language is so important in communication and is largely responsible for that "gut feeling" people have about other people.
I can make people steer clear of me with just a look or a tone,a stance.I'm a very loose person by nature but I'm also moody so it depends on the day and I've wound up noticing all this stuff over time. My face seems to tell my mood and especially if I'm in a shit people steer well a round me with a bit of an oh shit watch out for this dude look. My bad moods show even if I'm faking it.
I'm guessing with you that your not a very relaxed /loose looking/manner person tho, that one does it every time.
Another huge thing they pick up to is if your observative. People really notice that and get a bit worried that you'll see straight through them.