So the victims become the bullies. It's a vicious cycle. Bullies don't want to get caught. So you may never see a child intimidating another or know about it unless your students feel it's okay to tell you about it without suffering repercussions from the bullies. It is a myth that only boys are bullies.
Bullies usually have high self-esteem.
Both bullies and victims need your help. Zero tolerance for bullies must be your classroom credo. First, communicate that mistreatment or disrespect of others will not be tolerated. But remember to manage the classroom in a positive way. For example, catch students doing kind things for one another.