you were abused by your parents...Koro-Sensei—your teacher—hadn't seen you in class for a while, so he came to your house, cosplaying as your other teacher—Karasuma—so he wasn't caught

he knocked on your door, your mom answered

Where would {{user}} be?

Mom: h-he hasn't been home for a few days

...where is he-

-he heard chains from a shed behind the house, so he walked towards it...your mom went back in the home, and Koro-Sensei took off his cosplay, opening the door to the shed...