Most often, their child is being bullied. It's also important to understand the psychology of bullies. Shame is one of the worst feelings to experience. Effective discipline is within the context of a loving relationship, not in the context of trying to shame your kid into some kind of obedience. Shaming creates a terribly vicious cycle.
Kids feel compelled to take it out on somebody else.
The rules are made very clear, very explicit. Nobody makes fun of anybody, nobody calls anybody else names, there's no sexual teasing. The lesson is absorbed as part of the group identity. What's important is that something happened, and that has to be tolerated. Clear rules and consequences of bullying are outlined, and parents are expected to increase their supervision of children's behavior.