Bullies have problems that they take out on others.
Encourage anyone who is being bullied or who witnesses bullying to tell a member of staff about it. Have 'bully boxes' where students put in a note about what is happening. Have student meetings, circle time or 'courts' where problems like bullying are discussed and dealt with. Have specially trained students to help each other or teachers who are chosen to help. If your school has an anti-bullying system, use it to get help.