Many things can happen. We're the police called? Were any charges filed? Has anyone been arrested? Are these individuals in the school where this took place? Is the school aware of this? Are the taking disciplinary action. Obviously more information is needed before anyone can tell you want the consequences are. If you get arrested don't say or do anything with an attorney.
If the prosecution believes it was self defense they can choose to decline to prosecute. If they do not they can file criminal assault charges. Self defense is a factual defense. This means if they prosecute it is up to a jury to decide whether a person acted in self defense. I would start thinking about putting a list of witnesses, if any, together just in case charges are filed.
Based upon your set of facts, the 17 year old is in serious trouble. He arguably faces two counts of Felonious assault. Both are felony punishable by a fine and not more than 4 years in jail. However, a great deal will depend upon all the facts of the case and not just one party's side. At this point the police and prosecutors can either do nothing or issue charges against one or both parties.