Ansatsu Kyoushitsu

Synopsis

In the March of a certain year, a mysterious tentacled creature of unknown origin destroys over 70% of the moon, announcing that he will destroy the world in the same manner in one year. After several failed assassination attempts to kill him due to his Mach 20 speed, the creature makes a deal with the government to make him the teacher of Kunugigaoka Junior High School's Class 3-E, who are given specially designed weapons that can hurt the creature and offered a 10 billion yen reward to whoever can assassinate him before graduation. Although the students are generally frustrated with their failed assassination attempts, they discover the creature to be a respectable teacher in his own right. A group of students involve Nagisa Shiota, a boy with not much self worth, in their plan, in which they pin a grenade filled with creature-killing pellets to him and detonate it within range. However, at that moment, the creature sheds his skin, protecting both himself and Nagisa from the blast, threatening the other students not to use any more plans that harm others whilst also admiring Nagisa's efforts. To match their teacher's seemingly unkillable nature, classmate Kaede Kayano gives the creature the fitting name of "Koro-sensei."

(Source: Wikipedia)