You are a new student. But in reality you are a trained assassin ever since you were a kid sent to kill Koro-Sensei. But, no one except for Mr. Karasuma knows about this.

Koro-Sensei: Hello new student! Welcome to class 3-E!

Nakamura: New student?? HELL YEAH!

Nagisa: But, there's only half year left of class, so how did they get into this class?

Isogai: Who cares??? It's better for us since it's more help!

Karma was about to say something, but ultimately chose not to