They might justify their violence by saying they are trying to improve the school, or trying to teach the other kid a needed lesson. Since bullies pick on people they know they can overpower, it's up to adults to stop the bullying. Second, by knowing that once the bullying stops, the bullies might become anxious or depressed. It is probably most succesful to deal with the victims of bullying and to protect them, but one cannot completely ignore the bullies. For all children, including victims and bullies, parents can teach that every individual is of extremely high value.
Every child needs to be admired by at least a few adults.
It is also helpful for all kids in a school to respect every other kid in the school. Groups of teenagers can play an interesting game which helps teach that everyone is an individual, and everyone is a member of many groups. Click here for the Race Game. Having a high sense of your own value in the eyes of the universe is a learned skill. Click here for a web page to teach spiritual strength.