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I think that there are many options to available to you. Bullying is a very hot topic in our schools and many national media agencies are looking at this problem. One of the things you might concerned is going to the school counselor and/or principal and ask if they can bring bullying information into the school. There are also mediation services available through the school were the counselor and involved parties can together and talk. I am glad that you are hearing your child and taking her/his concerns seriously. are your child's strongest advocate. Be courageous, get informed, and be prepared not only to help your child but also the bully. I hope this encourages you to seek the assistance of the school personnel, they are there for the well-being of all the students.
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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)XXXXX XXXXX, 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.
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