Part of being a teenager is defining oneself in relation to one's peers. This is a process that your teen will go through on her own, but there are some things you can do to make it easier.
You're not going to have much to do with whether she eats lunch with Mary or Susie, but what you can do is try to build her a platform from which to operate.
The best platforms encourage adolescents in their special interests. You may have a talented teenager who excels at the piano and loves it. His love of music and ability to play well can get him through rough times with other things.
Some parents start collecting show tunes if their teen collects show tunes, or start gardening if that's what their teen is into. That's usually the wrong thing to do. It's important that your teen “own” the experience independent of you. She's creating her own existence.
But the teen years are ones of transition, and many times the young piano player enters middle school and promptly quits; or the boy who loved shooting baskets decides he's never going to grow and refuses to try out for the basketball team.
If up until now your teen has derived great pleasure from an interest, take time to open a discussion on the topic. It's a shame when an adolescent drops something he's terrific at, and perhaps there are ways you can make this break a short vacation rather than a permanent rupture. Or perhaps you can change things to make it better.
For example, the young basketball player who is suffering because he's so much shorter than his peers might be encouraged to stick it out one more year because he'll surely grow. And the piano-playing teen who wants to quit because she doesn't like her music teacher might be encouraged to continue if you include her in the search for a new teacher.
Otherwise, encourage him in other interests. Whether it's baseball-card collecting, tinkering around with the computer, or collecting jazz CDs, encourage it.
This also offers you an avenue where you can enhance his experience. You can bring home special music magazines, or offer to drive him to a baseball-card convention (if he doesn't yet drive). By showing support and interest, you do a lot to increase his self-esteem.