I went to school in the 90 and 2000's and while a lot more was tolerated this was not a fight. This was a guy running up and assaulting someone who was completely unaware and defenseless from behind. I think its pretty obvious the difference between this mugging and two willing combatants with someone getting their A$$ kicked.
I think this has to go to the legal system and maybe its just him being kicked of the track team, a suspension from school, and some type of probation that gets cleaned off his permanent record if he meets some conditions. If he has a record already its probably more severe. But you can't just let assault occur. Individuals who do this type of thing generally do not suddenly behave better if left off the hook.