Teachers can do a great deal to prevent bullying at school. First of all, they can help to build up a good community among the students. A good community is the most important factor to prevent bullying as the students are one group instead of many small groups. I've seen this happen back at school. My class was the only one without a good community, and we were the only ones with bullying.
Of course, as a teacher, you can't always manage to prevent bullying at all, but even if you think you are helpless, you are not. There are so many ways to strengthen your class's cooperation and community as well as each individual. Here are only a few tips:
Help your students to discover and develop their personal strengths.
Promote cooperation instead of rivalry, e.g. by doing teamwork exercises.
Offer class activities outside the normal school activities, e.g. visit a museum together.
You know your class best. Think of activities which could be fun for all of your students. Why not try to play a short theatre play for the parents? It could easily be integrated into English classes, is fun (for most students at least; consider your individuals) and promotes cooperation.
When you try to use these strategies from the beginning instead of waiting for first signs of bullying, you may be able to prevent bullying completely in your class. And honestly, this goal is worth the extra effort, isn't it?