Schools want people to report any problems they're having with bullies, etc. Usually there are warning signs before physical things happen.
Schools are not courts of law, and it's not for them to decide who was acting in self-defense, and who started it. The school can get into trouble for any physical conflicts permitted. They need to have a zero tolerance policy.
If they tried to permit "self-defense," the bullies commonly would set up situations where they could claim "self-defense" to get away with it, when they're actually the ones causing the problem. And the school doesn't have the processes available for hearings, etc., to sort it out.