There's nothing wrong with being an introvert... but there are degrees of introversion. And self-study is the secret to learning, which you seem to understand, but you might be surprised to find that you self-study better when you've got a team, a goal, a project, or a group. If the groups at your school seem to suck, there are other ones around. You've even got the Internet, which I didn't have at your age.
Here's one regret from my high school days: At one point someone came up to me and said "You're a big guy and you're not already on the football team or any other team. Want to learn to play the tuba and march in the band? We need more tuba!" At that point I was a very shy freshman, easily embarrassed, with no musical experience, and that whole idea sounded preposterous, like some kind of practical joke. So I said no. I wish I hadn't done that. It turns out that they were probably serious. And playing the tuba is probably pretty easy! I might have faked it pretty well with just four notes and a basic sense of rhythm! And it might have been fun to march around with the band... a nice, structured group activity with a lot of depth. (There is always a more complicated piece of marching-band music out there...) And I'd be have been that much farther ahead on learning something about music.
Hang in there. Don't drop out of high school if you can possibly help it -- it's good to have the degree, and believe me it will end. And college is a big improvement!
 They pulled me and a few others out of freshman algebra class and just gave us the AHSME, one day. I had no idea what the AHSME was, but I took it, and I ended up in a three-way tie for first place in my high school with my best friend (another freshman) and a senior member of the math club. I learned about the math club the next day when its faculty adviser came stalking into the cafeteria during lunch, waving the AHSME results in one hand, with the wild-eyed look of a prophet on a mission. He approached me and my friend and more-or-less told us that we had to join the math club, it was vitally important, because we had the knack.
That guy was great. Teaching is a weird business. 99% of the time you are saying the same things that you always say, but the remaining 1% are tiny moments which change people's lives forever.
My mother _forced_ me to try out for drama. She said 'all you have to do is audition.. then I'll never bring it up again'. I can't tell you how much the thought of even going to the audition repulsed me, much less performing on stage.
At the audition, while waiting, I was hanging around the most friendly people I had ever encountered (ok, it was a bunch of girls that were actually talking to me). Suddenly the fear of being onstage was less than the fear of being lonely.
I went from being scared of walking down the hallway to being able to talk in front of large crowds. My Friday nights of soldering and programming turned into a different kind of fun.. a chance to socialize with others.
(btw I can't sing for the life of me.. the trick is to stand in front or next to people with a similar voice. If you have a decent director they'll do this automatically)
Math club heaven. If only the Web had existed when I was a teenager.