Saijaku Muhai no Bahamut

A former prince of the fallen Arcadia empire, Lux, stumbles upon the bathing area of a female dormitory and witnesses the new princess, Lisesharte, in a state of undress, prompting her to challenge him to a Drag-Ride duel. Drag-Rides are ancient mechanical weapons unearthed from ruins worldwide. Once known as the strongest Drag-Knight, Lux is now labeled the "undefeated weakest" because he refuses to strike in battle. Following the duel, he is enrolled in a female-only academy that trains royal heirs to become Drag-Knights.