This is an important question. There are several things that can be done. Let me list a few of them.
First, education is important. Teachers need to discuss why bullying is wrong. They also need to teach on topics, such as racism, sexism, and classicism. By discussing these topics, children can learn to be more tolerant towards each other.
Second, the school also needs to teach parents about bullying. Sometimes the parents know less than their children. In light of this, there should be a handbook for parents that speak of a zero tolerance policy when it comes to bullying.
Finally, if bullying takes place, the school needs to have policies in place to discipline students. Consistency in this area will help to create a culture where students take seriously their actions.