Koro Sensei Quest!

殺せんせーQ[クエスト]! (ONA)

Spinoff of Ansatsu Kyoushitsu and continuation of the anime film short. In this world, Koro-sensei is the Demon King while the students of class 3-E are cast as heroes who must defeat him. The students learn swordsmanship and sorcery at Kunugigaoka Magic School. Their strange adventure begins! (Source: Official page)