with your son being a Green Belt - you should talk to his Grand Master or Saa Boo Nim (Tae Kwon Do) and see what they say is the best course to handle it..
My son, 12 is a 1st Don Black Belt. So I GET that he was defending himself.
I would ask him what the fight was over and who threw the first punch. To me, that would be important. Get to the REASON why the fight happened. Teenage years are tough. I know with my daughter it was and she was only with me part of the time...the testosterone in boys is high sometimes...
I am REALLY sorry that this happened. I am sorry there are people on here are trying to tell you what to call your son...my kids will ALWAYS be my babies... I don't care if they are 40 years old...they are still my babies..so you go girl!!
now back to your son...take pictures of his injuries. Then call your doctor's office and see what they want you to do...do they feel he needs to be looked at via X-rays or something??
Talk to the school principal Monday morning. the kids wearing "rings" is in the wrong...those are considered weapons and there aren't supposed to be weapons on school grounds.
Have your son write down EXACTLY what happened. Scan it in the computer so it can't be changed and send it via e-mail to the principal as well. I would be VERY disappointed if he had Varsity friends who did nothing to stop it. That would bother me - A LOT.
The important thing is COMMUNICATING with your son. Let him know you have his back and keep the lines of communication open with him...
HUGS TO YOU!!