ANDY'S ANSWER: I think I would like to know the exact circumstances of the bullying and how you are defending yourself before I say it's ok. For instance, if a bully trips you, is it ok to take a baseball bat and whack him along side the head? Um, no.
If someone is hitting you, can you grab their arm and secure it behind their back? Can you hook your leg behind their heel and pull them down? Yes.
If someone is calling you names, can you say, "I know I am, but what are you?" Yes. Can you spike their chocolate milk with a bar of Exlax? No.