Become an expert. One way to make friends is by sharing a common interest. If your teen likes in-line skating, have her learn as much as she can. Then when others talk about it, she'll feel confident in joining in. Join a club.
Many schools have clubs that deal with everything from ecology to computers to chess.
If your teen meets people who share her interests, she'll make friends more easily. Take a risk. Encourage your teen to make the first move. Talk to the person who sits next to her in algebra. Ask a question about an assignment.