Hero Classroom

Long ago, a powerful demon lord ruled over the people until an equally powerful hero rose up to defeat him. To counter future threats, Rosewood Academy, a school for heroes-in-training, was created. Today, Rosewood Academy enrolls only the best of the best, and Arnest Flaming is the best of them all. The top student in the school, a dutiful girl nicknamed "Empress of Flame", she has an irritating encounter with a light-hearted boy in the school hallway one day who seems to equal her powers, though she's never seen him before. He introduces himself only as Blade, and Arnest soon finds out not only is he transferring into the school as a new student, but she is personally requested by the King to help him settle in to daily life in Rosewood Academy.

Original Webtoon

When the hero Blade defeated the demon king, he lost all his powers. For him, this should have been a dream come true—finally, life as a normal student without fighting or adventuring. That is, until he discovers his school is a school for heroes. He just wants to get by with a solid average, but as a hero with real experience, it turns out his idea of average is a little off...! (Source: Yen Press)