Rabbi Dr. Twersky deals alot with this issue in his books and articles. He emphesizes that to overcome a bully you need to be assertive about the other person’s wrong doing WITHOUT coming across as being over emotional. When the bully sees that his pranks or insults do not ellict the response he/she wants, they move on to next person!
I have worked with young boys before and i have seen this…the good boys who don’t get tormented by the bullies, are those that are assertive and nonchalant…they simply say “whats your problem? go bother someone else” or “I don’t care what you have to say, your words/actions don’t bother me…”
It may take some time and a few unsuccessful attempts from the bully side to get the response he wants, but once bully realizes that he/she won’t get it from that kid, they get bored and move on to a differen victim…
Of course a teacher and principal should be consulted and advised about this but
it is a very important skill to teach a child to stand up for themselves in a non aggressive and assertive way.