Silver Demon King

Reading manhwa Silver Demon King at manhwa web.

Summary:

Han Yu-an, a senior in high school, failed the SAT exams as he accidentally shifted down his answers on the answer sheet. Before him appears a handsome foreigner– no, a demon, who says he could turn back time for Han Yu-an. However, he would have to act as the stand-in for the Demon King… Can Yu-an safely return home after having this abrupt change of lifestyle?
“Should I just live however I want? Because until the real Demon King returns, I’m the actual Demon King!”

Silver Devil King, 銀光の魔界王, 은빛마계왕