Time to Get Down to Work

Time to Get Down to Work (基礎の時間) is chapter 4 of the Assassination Classroom manga.

Short summary
Korosensei watches as Karasuma trains the class to swing knives for Phys. Ed. class. Distracted, Karasuma tells Korosensei to play in a sandbox nearby. Sugaya reminds Korosensei that his lessons are impossible to follow anyway. Karasuma tells the class the importance of learning the fundamentals, just like with studying, to have even a chance of killing Korosensei. He demonstrates this by inviting Isogai and Maehara to try to knife him, only to outmaneuver both of them. At a end of class, Nagisa comes across a familiar face, Karma. Karma goes to greet Korosensei, then attempts to assassinate him with knife blades hidden in his hand. As Karma intimidates Korosensei, Nagisa explains to Kayano that he was suspended in second year for committing numerous acts of violence, and for that, admitted to class 3-E.