Silver Demon King

Yoo-An Ha, a high school senior, messed up his SATs because he mismarked his papers. He is approached by a foreigner- no, a demon who proposes to turn back time. In exchange, Yoo-An must stand in as the demon king's replacement... Will Yoo-An, living the demon king's life, be able to safely return to his own life? “Should I live however I want? I'm the real king until the real one returns!”

(Source: Copin Comics)