Bystanders and bullying
When bullying takes place there are usually a number of bystanders present. How they respond can determine whether the bullying continues or gets worse.
Teachers can work with classes of students to increase the likelihood that bystanders will respond constructively. See greatergood.berkeley.edu/article/item/playground_heroes.
A useful exercise with students makes use of a brief questionnaire, followed up with classroom discussions of how they think they would respond as bystanders if they observed someone being bullied. There follows a questionnaire you might like to use and instructions on its use.