I would never become a teenager again, not if you paid me a million gazillion dollars. Never. NEVER. (Did I mention "never"?)
Teens are so sensitive about everything, but they don't realize that part of it is their stage of growing and not something irrevocably wrong with them and the world.
What does your daughter like to do, in school or out of it? If there's something she likes and is good at, encourage her to get more involved in it. If it can apply to a school activity - such as her love for small children leading to joining a future teachers' club, her love of reading to helping in the library, or her work in piano to accompanying the choir - encourage her to join. It's worthwhile to try out activities, because they can turn out to be interesting even if the people aren't. It may take courage for her to join an activity, but courage is what it's all about.
Another thing she can do, which also takes courage, is not to wait for people to be friendly to her. She can smile and say hello to everyone... (keep on because this next is important) ... not expecting ANY response from anyone but doing it anyway. It's good practice for the grownup world, and lots of kids are hesitant to do it - they hope somebody will notice them first.
I was in high school back when dinosaurs were roaming the earth, but I was very shy, and I clearly remember obsessing (that has to be the word) about what other people thought of me. I had no idea they were worrying, too - I thought I was the only person who didn't have my act together. I was always better when I was BUSY and didn't have time to think about myself.
P.S. Just read the other answers! Mamapedia gals are the greatest! Momma L.'s post moved me to tears.