Ansatsu Kyoushitsu

Synopsis

The students begin their assassination attempt by showing Koro-sensei a movie exposing all of his embarrassing habits, weakening his mental state while also distracting him from the chapel slowly filling with water and bloating up his tentacles. Once the students shoot off the tentacles they won in the exams, they surround Koro-sensei with a hydraulic cage and block off his movement, allowing the snipers to shoot at him. However, Koro-sensei activates his ultimate defense, trapping himself in an indestructible crystal sphere for the next 24 hours which, despite limiting his movements, leaves him completely invincible. As the class lament their failed assassination, a number of students suddenly start showing signs of severe illness. This is revealed to be the work of a mysterious third party who had spiked their drinks with a supposedly deadly virus, demanding the remaining students and teachers bring him the immobilized Koro-sensei in exchange for the antidote. Deciding against the trade-off, Koro-sensei and Ritsu instead come up with a plan to infiltrate the high-security hotel and obtain the antidote.

(Source: Wikipedia)