Mutual combatants rule?
Not likely.
Once the assault by the others ended, and he drew a firearm and shot, he was now the aggressor, he could have fled, sought protection from adults, etc.
The prosecutor likely is going to have ‘splaining to do. The judge too.
Once an assault ends, any re-igintion is a new incident.
I doubt the others said they were going to find him later etc.
Stinks of something.
Rules? Would Marquis of Queensbury apply?