Hum... an assault is a criminal act. Call the police and have the criminal charged. If others seek to cover it up, whatever, have them charged as accessories. Sue the school administration for failing to act appropriately in providing care for the victim. Seek money damages. Use an attack dog grade lawyer. Money is the only thing these people will understand. So, give them a message in the language they will understand.
In a perfect world, that is how it should work out.
But in reality, there are far too many factors that preclude that response. My main tormentors were the kids of illegal aliens. Half of them did not speak much English- but they joined gangs easily enough. No parents to contact and to contain those kids. Not to mention..... PC feel good contortionist twisting by school admins made it as though I was being cruel and unfair to the little migrants by using my FACE to hit their hands with.
When it came to my son, I made it really clear that there would be horrible consequences for the kid if he ever made a move at my son again, and his father and mother both understood that I was going to hold the father personally responsible for his son's actions- and my son's response was equal to his size- about 1/3 of my size and my response.
My dad was in a really strange position; when he came to the school one time because I "was picking on 4 poor 'lil kids" who could not even speak English to avoid a confrontation with me- my dad was told it would be racist to press charges and involve the police, etc.
He told me, pretty much I was on my own unless he was around, so make sure that if I wound up on the ground with one of them, make an example of him so his friends would think twice before coming after me.

If you saw where my dad grew up, in an orphanage, you would understand that he knew where I was coming from, and experience had taught him the same lessons he was imparting to me. HA! He had moved me from that place in Hawaii, to SoCal and we wound up in a nicer neighborhood; the problem was my school was in a rathole that I had to go through each day.
Oh well- he sure tried!