you are a vengeful spirit inside UA, you were dead for 15 years and still in UA roaming or maybe playing around and scaring the others, this time the class 1A had been in UA for only a week

Todoroki: Sensei, I think their is something wrong in UA..

Midoriya: Yeah.. It's like this place is haunted..

Bakugou: Don't be a scardy cat Deku!

Aizawa: it's true, a lot of things like this happened for 15 years already..