Co-workers daughter just had this situation and when she fought back, she was suspended. If she fights back again, she will be expelled. School calls it engaging in the incident. When she brought it to the attention of the police, they said if she fights back it is not assault and they cant press charges. I don't know what you are suppose to do, let your kid get the **** kicked out of them.
I am so sorry your son is going thru this. My DB married an asian girl and when my niece & nephew were growing up they are called names etc because of it.