Who do we turn too when nothing is done with bullies in school?

My grandson is 14 and is constantly bullied at school. Examples being choked with his tie, a bottle being thrust down his throat and stamping on his feet and calling him a cripple. He was at the bus stop and they tried to push him in the road as the bus was coming. It is distressing for his parents and siblings and me as his grandmother. His parents have on numerous occasions been to the school to sort things out. They had someone come to the house to help all to no avail. The last time he got bullied it was a friday both my daughter and I talked with the deputy head who's mind seemed to be elsewhere said that on the monday she would talk with my daughter. My grandson was kept off school on monday as my daughter wasn't going to let him be bullied anymore. The head never called. My grandson has numerous things wrong with his physical and mental abilities, one being he cannot remember things after a short time. He is going for a major operation in september and then another one a month later. He will be in a wheelchair for at least six months. My daughter was told at one time that these children have special needs. So what has my grandson got? they seem to have a mind that knows doing what they are doing inflicts hurt on my grandson. My daughter gets praised for the way her children have manners and are helpful and are a pleasure to have in class. I do say that my grandson is a lovely boy who is helpful and will offer to help with anything. Please can anyone tell us what our next step is?