No parent wants to discover that their child has become the school bully. Of course, the responsibility comes down to the parents, and you are the first line of defense.
For starters, you can have your child apologize to the children involved. Make sure that the parents are involved on both sides. Explain to your child that each person is different, and focus on the impact that bullying can have on those around them. If you can get to the root cause of the bad behavior, you can also address these problems. If you notice that bullying remains a problem, you can also arrange meetings for your child and a therapist.