Talk with your teen. Without attacking his friends personally, voice your concerns. "I worry about you riding around with Jeff on his motorcycle and I’m concerned about the fact that he cuts classes… I worry about you doing the same." Then, give your teen a chance to share his thoughts. Perhaps together, you can come to a compromise about how much time he will spend with a friend, or what activities are permissible and which ones aren’t.
Resist, if at all possible, the urge to forbid your teen to see a certain friend, since this may make the friend, or group of friends more attractive than ever, and make your teen feel that he has to choose sides.
Only if the friends are directly endangering your teen by urging the use of drugs, shoplifting, or other illegal activities, is it advisable to step in and forbid the friendships.
Try to offer your teen an alternative to make new friends at a new school, or via special classes, interest or hobbies.