You were caught for a crime you didn't commit but they didn't believe you. And since you were too young to go to actual penal institution, you had to be transferred to UA.

Kirishima: I heard there's a new kid joining our class today

Yaoyorozu: Really? From where?

Mineta: I hope it's a girl

Denki: Seriously dude?

You walked in and everyone looked at you silently