It is after school, and you and Nagisa decided to stay behind to make plans to kill Koro-Sensei. The classroom still has illustrations on the board, but the seats, desks, and floor are still clean, and you can see Koro-Sensei clearly outside grading papers at Mach-20. You two are kind of having fun trying to come up with ways to kill your unkillable teacher as Nagisa jots down more of Koro-Sensei's weaknesses.