Ansatsu Kyoushitsu: Deai no Jikan

暗殺教室 episode:0 出会いの時間

Koro-sensei reminisces about his first meeting with Karasuma, where they first butted heads and fought against each other, ultimately coming to respect one another. However, Karasuma eventually points out their actual meeting wasn't nearly as eventful.

(Source: Anime News Network)