If the environment at your school supports bullying, working to change it can help. For example, there may be areas where bullies harass people, such as in stairwells or courtyards that are unobserved by staff. Because a lot of bullying takes part in the presence of peers (the bully wants to be recognized and feel powerful, after all), enlisting the help of friends or a group is a good way to change the culture and stand up to bullies.
You can try to talk to the bully. If you don't feel comfortable in a face-to-face discussion, leave a note in the bully's locker. Try to point out that his or her behavior is serious and harmful. This can work well in group situations, such as if you notice that a member of your group has started to pick on or shun another member.
Most people hesitate to speak out because it can be hard. It takes confidence to stand up to a bully — especially if he or she is one of the established group leaders. But chances are the other students witnessing the bullying behavior feel as uncomfortable as you do. They may just not be speaking up. Perhaps they feel that they're not popular enough to take a stand or worry that they're vulnerable and the bully will turn on them. Staying quiet (even though they don't like the bully's behavior) is a way to distance themselves from the person who is the target.
When a group of people keeps quiet like this, the bully's reach is extending beyond just one person. He or she is managing to intimidate lots of people. But when one person speaks out against a bully, the reverse happens. It gives others license to add their support and take a stand, too.
Another way to combat bullying is to join your school's anti-violence program or, if your school doesn't have one, to start one of your own.