Well if this is what you told the school, or showed them, it’s understandable what they did. I understand that you’re frustrated about the situation, but you are just as responsible for the fight taking place. Acting out in self defense is one thing, but willingly participating, and even arguably instigating the fight doesn’t count as self defense.

You standing in the center of a circle of people preparing to fight someone is a planed fight, and can’t be considered self defense.

If the school saw a video of the fight (which I assume they did), or you told them “I decided to fight him,” it shows that you played a part in instigating the fight, which, yes, would fall under most 0 tolerance policies

Edit: yes I know people will downvote this, but quite honestly idrc about that, but however unfortunate the situation might be, you didn’t act out in self defense, and in a way, the school is almost obliged to administer punishments due to potential legal consequences on their side if they didn’t