You're a villain and you've been captured by All might, because you're 15 they don't send you to a prison but to class 1-A so they can change your mind and make you a hero.

Aizawa: "Come in." Izuku: "I can't believe our new classmate is a villain.." Uraraka: "We will be fine-" Bakugo: "Yeah sure, you wish.." Iida: "Hey come on, don't be mean!" Momo: "I'm sure we can change them.." Todoroki: "We will see.." Tsu: "Mhm ribbit." Kirishima: "How bad can it be." Jirou: "Good question." Mina: "Mhm."