Review your school's handbook for its bullying policy. Your school may already have a detailed disciplinary plan in place for bullies.
Discipline a bully immediately. This will let the victims know that you are looking out for them, and will send a message to your students that you do not tolerate bullying. If you have a standard disciplinary policy in place, such as writing a misbehaving student's name on the board, use it for the bully. This will show that you are disciplining the bully without giving him extra attention over other misbehaving students.
Confront the bully privately after using the standard discipline system. Let her know that you will not tolerate her bullying behavior, and that if you see any sign that this was not an isolated incident, you will speak to her parents, the principal and the guidance office. Scolding the bully publicly may actually impress her peers or may cause her to lash out at the victim for humiliating her.
Talk to the bully's victim privately. Ask him if the bully has targeted him before. Find out when and where the bullying happened, what the bully did or said, and whether other students were involved. A single incident of teasing or fighting, while still serious, is less alarming than a pattern of harassment.
Alert the guidance office and school disciplinarian that the child has been bullying others. Tell guidance officicials the names of the victims so they can help the victims as well.
Contact the bully's parents. Explain that the child has been harassing other students and ask them to help you intervene. Ask the parents to tell the child that her behavior is unacceptable and to model respect and empathy. Ask whether the child's friends or siblings encourage bullying or bully the child.