I truely feel for you. I know you are unable to change schools and your kids have to deal with the situation. My son was also being picked on by kids. This was more of a one on one picking, not a group of kids like yours sounds. We told our son to go through the proper steps and if that doesn't work, he will have to take matters into his own hands. I hate to admitt that but NO ONE will pick on my babies!!
My son was getting pushed and punched in his privates. We finally told him that when that happens, to grab the boys wrist, give it a twist so it hurts the kid, then look him straight in the eye while he is twisting the wrist and tell the snot kid "Do that again and I'll brake you wrist". I know some parents will not agree with, but I am here to protect my kids, expectally when the grown ups don't or won't. This did work for my son, and the boys never touched him again. I also told my children that if they do have to stand up for themselves and protect themselves and DO get in trouble, we will take care of that as the time comes. They also there are rules, even with bullies. They are never to hit first and my son is never to push or hit a girl! We are also taking Tao Kwan Do for self defense and self esteem. My kids have good self esteem, I just want them to stay that way.
Good luck with your kids. If you start asking around, I am sure you will find other kids that are having the same problem and if the parents can get together and approach the school, they will have to act faster.