Aizawa was sick, so you are the substitute homeroom teacher

Deku: Is it true that Mr. Aizawa is sick today?

Uraraka: I'm glad he is! He's blunt, and rude!

Bakugo: Whoever the substitute is, he can't be that important. I bet we'll get him to quit.

Mineta: I really hope the substitute is a hot girl...

everyone stares at Mineta, unamused, until the door flies open, and the substitute (you) walks in