You have my sympathies entirely as do the others on here who have experienced the same.
It came to my attention just before the school holidays started that my 10yr daughter was being bullied by 5 girls in her year. She has mild special needs and has a slight walking/running problem as well as gross & fine motor skill difficulties. I went to see her teacher, who immediately took steps with regards to getting these girls to write apology notes to my daughter and her grandma (they were kept in at breaks & lunchtimes for 2 days to do this). I declared that I felt this was not enough and I wanted the girls parents to be notified of the incidents. The teacher told me this was "Step 1" of their procedures. Still not happy I took it to the Head Teacher, who took my concerns on board, and as a result contacted all parents involved and told them should these girls continue in September then they will be excluded from the school. Also the girls' new teachers have been informed of the situ and are supposedly on red alert to ensure it does not recur.
If you are getting nowhere with the teachers, ask them for their their bullying procedures (all schools should have one) and ensure they have taken the steps outlined in it. Personally, what I would do if I still wasnt having any luck and your child was extremely distressed, is to take them out of school and inform your Local Ed Authority & Ofsted - and tell the school so. Every school fears Ofsted and it goes in their reports on how they handle bullying. Maybe even expose them in your local rag too. Somebody would do something about this on the strength of such action - it will kick up a huge stink and you will get a result. If you do not wish your child to return to the school in question, my bet is you could take your pick of any school in your area (incl those extremely hard to get in to) - Good luck