I'm sorry to hear that you're having such a hard time coping. I can relate to you because I too have suffered from low self-esteem my whole life and I've also always been a loner. I was painfully shy as a child and in many ways I still am. I'm 39 now so I've been where you are and then some so I can relate and hopefully my advice and insight will be helpful.
First, I just want to ask if you're in counselling and/or on anti-depressants? If not, I would strongly recommend that you seek both. My family doesn't understand what I'm going through either and they don't understand how debilitating depression can be. That's why you have to seek professional help from someone that understands the condition, what you're going through and can help you work through it. Also, disassociate yourself from people that are negative if you can because it will only serve to make you feel worse.
Second, I just want to say that you shouldn't try to be like the preppies, the people who picked on you and that you grew to despise. You have to be your own person and it's your personality that will set you apart - not the clothes you wear or trying to fit in with the preppies or other like people. You have to stop worrying about what other people think. It took me a long time to learn that. Your real friends will be accepting of you no matter what. I don't know if there's any medication for Social Anxiety Disorder (I have that too) but you might want to look into that as well.
Third, you shouldn't really be concerned at this stage with getting or having a girlfriend. Although it's nice to have someone you have to "fix" yourself and feel good about you first before you can have a healthy, stable relationship with anyone. And, when you are ready to find someone she doesn't have to be "pretty" (read: gorgeous). I realize we live in a superficial world but personality is important too. Pretty girls can be shallow and mean just like those preppies that treated you bad. Try to find someone that is going to like you for you and be compassionate and caring - not materialistic. You're too shy to approach girls and believe me, I understand this all too well. Well, maybe try striking up a conversation with someone online(?) Let them get to know you for you; your personality instead of "what do you look like?" blah, blah, blah. Don't go into it thinking "potential girlfriend" though because you'll just end up feeling down all the more if things don't work out that way. You might just make a good friend and then who knows? Maybe your feelings for each other will grow.
Fourth, if you live in a city of any appreciable size they must have public transit. I understand that you'd much rather have your own transportation but for now it may be what you have to do until you can save up some money to buy your own wheels. For now, get a job at a fast food restaurant if you have to, just so you can start saving some money for a down payment on a truck or whatever. I'm impatient when it comes to things like that too. I expect everything to happen overnight but the reality is that things just don't happen that fast. If you're patient, persist and work at it it WILL happen.
Fifth, don't let those court-ordered people dissuade you from getting your GED. Also, the teacher should ask them to leave if they're being that disruptive in the first place. You'll feel so good about yourself knowing that you accomplished what you set out to do. These days, you need a high-school diploma or GED to apply for most jobs anyway so you may not have a choice but to get it. Get 'r done!
I hope my advice and insight has helped a little bit. I'm no Anne Landers - just someone that can relate to how you feel and to let you know that you don't have to go through this alone; there is help!