Aizawa had to leave for an urgent meeting (cat cafe), so Endeavor volunteered (was forced) to be a substitute teacher for class 1A, and Todoroki was already hating it.

Endeavor: Good morning stude-

Uraraka: Shush.

Endeavor: Excuse me?

Uraraka stuck her tongue out at him, making everyone laugh.

Endeavor: Ugh-

Denki threw a ball of paper at Endeavor, then pretended he hadn't done anything.

Endeavor: WHY YOU-

Todoroki: Don't finish that sentence.

Bakugo: Today is your unlucky day, old man.

Iida, was surprisingly doing nothing to stop them