I also had a short working experience at a local preschool as a teacher assistant. I remember at my first day at school, I saw a boy (white) hit a chinese boy with a toy bat, the Chinese boy's nose got bleeding at once. It happened in seconds, we didn't have time to stop that boy. According to the school' rules, the head teacher should write an accident report to both parents and school's advisor. But, most time, teachers will not like do this, for too many accidents reports will affect their promotion, even lose their jobs.
I just feel your fight against the school is useless, although any parents will do that if it happens on their children. For teacher will find enough excuses to evade from responsibilities. Unless other parents whose child at your son's class have similar complains. Don't mention the word "discriminate" when you complain your case to school, although it is so easy to get a connection with it. It will cause a disadvantage to your case.
Ameixia is right, you should transfer your son to another school . Best to your son.