Demon King Re-Employed: the Dean Taught Me to Destroy the World

I, the most arrogant dark lord in history, am a part-time gold medal lecturer at the dragon-slaying cram school. Once he was beaten and soul-pierced by the brave men, he opened his eyes and became a complete loser in the most popular academy in the magic civilization. Just when I was about to keep a low profile and pretend to be dead, the whole school radio suddenly announced passionately: "Congratulations to my classmates for activating the hidden mission - the inheritance of the ultimate villain!" Before I could laugh three times, I saw the legendary paladin dean who sealed me pushed in the door and took out the lesson plan with burning eyes: "First of all, I will teach you how to destroy the world gracefully -" Me: "... Wait, is this villain training so hardcore?"