Always a tough situation when your child is in trouble of any sort.
We have a special needs boy and expect these bully problems to arise in the future. We have had friends go through what you are going through and had similar slow reactions from senior school staff.
The main thing with anything like this is to be coming from a strong position. Our chosen strategy should the need arise, will be to send out boy off to school with something like this
https://www.google.com.au/shopping/p...d=0CM4CEKYrMBI
Should you use this option the important thing to remember is not to induce a situation for the purpose of using it. The situation has to what has been happening normally, not encouraged.
As had been said earlier, kids at this age can be mean, and their is normally a ring leader who is trying to be the most popular that everybody wants to be around.
Good luck with it, best wishes.