He warned her... Multiple times... Was this justified?

This was fully justified. She was warned.

Isn’t it interesting how everyone walks by and ignores the situation when she repeatedly harasses and hits him, but when he finally retaliates the whole school suddenly cares…

She laid hand on him first and she also tried to embarrass her. Let everyone keep their hands to themselves

Self defense is a human right, no matter your sex.

Completely justified and look how no one intervened UNTIL he defended himself then all of a sudden here come the white knights.

Could’ve just walked away but it’s justified

Short answer, yes it was justified Long answer, she and her friend made aggressive moves against him, he responded with reasonable force, and a bunch of cowards stormed forwards to basically tell him to take it when they should've stopped her in the first place.

Me trying to investigate the issue that caused the fight