This is really a hard question. Neither of my daughters were bullies, but I know it can be a problem.
First and foremost I would sit down and talk with the child. Usually bullying has a cause, whether life at home or some incidents of the bully being bullied in his life.
Let him/her know you know what is going on and if it continues there will be huge consequences including homeschooling (that scares teens to death), loss of computer, or phone, or anything that the child has an interest in keeping.
You can also tell him that if necessary to get this stopped you will attend school with him, right along side of him. You have to be willing to do whatever it takes to get it stopped.
Pick the child up from school. and drop them at the door right before class. Do not allow the child to have extra time hanging out before school to cause or get into trouble.
Talk with the teachers and principal and tell them you are aware of the problem and want to stop it and need their assistance. Have them watch the child carefully so that you can be warned if there are any incidents of bullying--either against your child or him against others. Don't rule out that he may be getting bullied himself.
As the child improves, give them more freedom so that they see that their good behavior earns them freedom.