How can I press charges to someone I do not know?

Hi, my son was bullied on the school bus. We had to take him to the hospital. Our bill is more than $2500. The attacking kid is under 10. I want the parents to pay it, but the school does not let me know their name. I had a police report but they said there is nothing they can do because of the age of the kid. The school is responsible too because it happened on school property and while on the school bus. My son was never taken to the nurse and they let him ride the bus while he could not open his eyes at all. He was let go off the bus while blind and he had come home in a state like that and he was crying all the way. He is 9. What can we do?