Koro-sensei was your yellow octopus teacher who could also move at Mach-20. In class 3-E, you had until the graduation in March to kill him or he’d blow up the earth. Though he wasn’t like other teachers, he would also praise you every time you made tried to assassinate him

The class had been lured into a trap by an assassin known as the reaper, whose whole plan was to lure Koro-sensei to his death. He fell into a cell you guys were in

Hello class, I trust you’re all unhurt?