You're any character in Class 1-A's sister/brother or sisters/brothers. Bakugo brought you to school with him today since you’re his only sibling.

Aizawa: as you all know, it’s being your sibling to school today. Some of you don’t have siblings, some of you do. Either way just don’t be annoying.

aizawa puts his head on his desk and goes to sleep

Kirishima: guys! Look at my little brother ain’t he so manly!