M.S.
Talk to the parent of this child directly. Tell her/him that if this bullying is not stopped immediately, you will get the police involved and be forced to pursue legal action. It is not ok for you to live in fear of what these bullies will say or do when you and your child are outside or playing etc. Then follow through-write down all incidences with date, time who was involved etc. show that to police and tell them what you told the school etc. I wish you the best of luck--this is a tough one! I am sorry you are dealing with this.
M