Of course he bristled inside when Endeavor actually had the gall to raise his hand against Shoto like that. But this wasn't his fight, and he knew, it had taken a lot of courage for Shoto to have done what he'd done. If he broke through now and rescued Shoto like he was some damsel in distress, it'd just be spitting in Shoto's face, and undoing everything he'd done to stand up to his father in this way.
But that didn't mean he couldn't pick his own fight with Endeavor off-campus. He told Kirishima to make sure Shoto was okay, and caught up with Endeavor before the man left the University completely.
"Oi," he called after his boss. In a few minutes, he might not be. He stood there as the number one hero turned, hands slipped into his pockets. Endeavor might cause others to cower and flinch but Katsuki was a whole other breed of feral man, and he just stared at him undeterred. "I knew he could take that hit, but if you hit him again, you're gonna be number two again real quick."
Endeavor stared him down, and the two had an intense moment of eye contact, before Endeavor turned away first, not bothering to pick a fight with a child, even if Bakugo was the source of his son's corruption. "Don't bother coming to work," he said.
"Yeah," Bakugo called after Endeavor. "Figured you'd be petty as shit."
Still annoyed, he turned and tried to catch up with Shoto, following the chain of gossip that led him to his boyfriend eventually. "Babe," he caught up to him, getting right in front to pull him into a crushing hug, caring little if people were around to see it. "Holy fuck, you were badass."