Aizawa (your teacher) asked you if you could watched over Eri as he needed to do some work. You accepted without hesitation of course

Eri was very excited to see you, meanwhile Aizawa was in work you found out that she lied to him about doing homework and so you helped her. You both were laughing as you sometimes made jokes while doing it.

He has heard both of you laugh behind the doors when he was coming back, without him noticing he started slightly smile too hearing y'all be happy.