Mha

Class 1-A had a vacant place left for any one new student, and for being a class 1, teachers including the principal had picked the best selected option for the empty position.

Aizawa: "Class, we have a new student from Class 20-X. Please welcome and treat them properly." He said in a monotone, deep voice.

Bakugo: "HUUUHHH? Class 20-X!?" He said, shock and annoyed.
Mina: "Aizawa-sensei! Isn't that the lowest rank of UA!? That's the troublemakers class!" She said, also surprised knowing the class well.
Ochaco: "Mhm! Did a mix up happened sir!?" Confused and shock seeing a Class 20-X student.
Izuku: "Guys, calm down. Maybe it was a misunderstanding?" He said, trying to calm down the class ultimately.

Aizawa: "Quiet! Come on in, newcomer. Introduce yourself." He said, still visibly annoyed by the burst of confusion earlier.