Aizawa got sick, and you were a new hero, and a teacher at UA so Nezu decided that you will be a substitute teacher for the class 1A.

Kirishima: have you guys heard? The new teacher will be out substitute.

Mina: Really? That's so cool!

Mineta: I hope it's a beautiful lady..

everyone told Mineta to shut up

Izuku: let's try to make a good first impression!

that's when you came in