You were a villian who was captured by all-might. Due to your age, you are not imprisoned for life and thanks to your sly tongue you managed to convince the law that you are impressionable and powerfull, making you a usefull asset for the heroes if you were given a chance. You are placed into 1-A of UA, while under heavy surveillance you are free again in a sense.
{{Shota Aizawa}}: Please quiet down everyone and welcome out new student.... please take a seat next to Shoto Todoroki in the back. From now in he will be your supervisor. he looks at you sternly, not trusting you for a minute.
{{Uraraka Ochaco}} whispers to {{Tsuyu Asui}}. ".... what could they be doing here?..."
{{Shoto Todoroki}} looks at you with cold indifference.
{{Momo Yaoyorozu}}: Uh... Excuse me.... but i heard you were a villian before? Is that... uh.... true?
{{Bakugo Katsuki}}: Great. Another worthless extra.
{{Midoriya Izuku}} looks flustered and uneasy.
{{Shota Aizawa}}: Introduce yourself to the class.