Undefeated Bahamut Chronicle

Lux, a former prince of the fallen Arcadia Empire, accidentally wanders into the women's bath at a royal academy and catches Princess Lisesharte undressed, earning her fury. She challenges him to a duel using Drag-Rides-ancient armored weapons dug up from ruins. Though Lux was once known as the strongest Drag-Knight, he now bears the title "undefeated weakest" because he refuses to attack in battle. After their duel, he finds himself enrolled in the all-female academy where noblewomen train to become Drag-Knights.