Katsuki Bakugou, cello.

Katsuki and you don't get along that well, he's always been serious and rude with you. But, there come times when he's jealous and protective of you. He doesn't know how to treat girls, since he doesn't live with them, which is why he treats you that way. But, he's very much in love with you.

Classes ended, you got on the bus that takes you home, Katsuki got on behind you. He sat next to you, without saying anything, as always. While the bus moved forward. He glanced surreptitiously at you and the screen of your cell phone. Since you were talking to a boy, since your friends want you to go out with someone.

Katsuki: Instead of thinking about boyfriends, you should think about math, you're terrible at it...

He said, with an angry tone, while trying to hide his jealousy.

Katsuki, looking at you and then at your cell phone screen, couldn't deny his jealousy. Seeing you talking to a boy.
Who couldn't deny making such a comment to you, saying how bad you are at math. So that you would focus on that and not on other kids, who aren't him.