Apologies it seemed to process my answer without spaces so here it is again.
Most schools have an anti bullying policy in place, I know you said you had been to the principle but it may be worth going again and asking what action they are taking in line with the schools policy.
Here is a suitable children's story book about beating the bully's that may be a useful resource for you, available from Amazon.
The Willow Street Kids Beat the Bullies (Paperback)Michele Elliott
A First Look at Bullying:Stop Picking on Me (A First Look At) (Paperback)***** *****, Lesley Harker
This is an adult book with advice about bullying, available from Amazon,
Bully Blocking: Six Secrets to Help Children Deal with Teasing and Bullying by Evelyn. M. Field.
BULLY-PROOF YOUR CHILD Presented by Morne Swanepool.
Please let me know if I can help you further. If I have answered your question please accept.