Why do schools suspend kids for self-defense against bullies, even when it’s unfair?

This is a follow-up to one of my questions. Yes, I've been trained to defend mysef for an early age; even taught how to shoot too if needed in a life-death situation.

I was once involved in an unfair situation with a boy in middle school. We were the same age (13-14 years old) but that boy was already bigger and heavier than me. I defended myself against a future predator after he purposely tripped me by sticking out his foot (while I was minding my own business, carrying my books) for thrills.

This resulted in a stomach strike and bloody nose for him. Here is the unfair part: While he got suspended for the whole week, I still got a 2 day suspension because I should've been the bigger person and reported it at first. I'm thinking, even if I was able to defend myself... that was still an unfair fight. He was unfair and cowardly. What is a big boy picking on a smaller, little girl?

Here is another unfair part:

That boy's parents were just as upset as my parents too. His parents even had the nerves to say ''He's bleeding, that little bitch throwing cheap shots, she just hurt our son, etc''. My parents obviously fought back verbally too. Even the boy's friends and other said he started it. Do you find it unfair that even if you're bullied and fight back (even a girl against a bigger, heavier bully boy... unfair... a fight we shouldn't ever get in), you still get suspended, still punished?

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I felt outrage for the school administrators to be honest. Off course I had to apply elements of surprise (what his parents called cheap shots) because a regular fair, I would've been destroyed right there. If someone much bigger is trying to hurt me, I'm not ever going to fight fair since it's already an unfair fight in the first place.
Why do schools suspend kids for self-defense against bullies, even when it’s unfair?
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