The Demon King and Class E

The Demon King and Class E (魔王とクラスE, Maō to kurasu E) is the first chapter of Koro-Sensei Q!.

Summary
A demon king begins class, prompting his students to attack him. However, they do not land a single hit and eventually exhaust their strength.

It turned out that a few members are revealed to have multiple irregularities with their equipment: Yoshida wields a plunger instead of a stick, Isogai is only wearing the front of his armor, Okuda is unable to control her magic and Kurahashi is only able to summon insects. These irregularities made the class the target of discrimination by the rest of the academy.

To their surprise, Karasuma, the captain of the royal knights gives the class a brief detail into their teacher's superhuman abilities and accomplishments, before announcing to the class that they had to kill him. This comes as a shock to the class, as they even lose to slimes (who are weak enemies). Karasuma then goes on to explain that these abilities are due to a phenomenon called "bugs" and that each student has them as well; The demon king adds that each of their irregularities are bugs (minus Kayano's small chest and Okajima's unwillingness to throw away porn magazines) and further reveals that he made a pledge to train the students up to the level that they can beat him, as he yearns for a strong opponent.

Later, the class heads outside and are assigned to fight a passing slime. Upon noticing them, the latter comically reacts to the demon king's presence. Nagisa Shiota volunteers to fight, which activates his bug; the result is always different when he steps forward with a strong will. In this case, his strength increases to incredible levels and allows him to beat the slime (with magic). Motivated, the class manages to defeat an incoming wave of other slimes, before making an unsuccessful attempt to kill the demon king.

As the class tries to follow their teacher, Kayano dubs the demon king "Korosensei" from the words "unkillable" and "teacher".