18+ Seireitsukai no Blade Dance

Kazehaya Kamito has been summoned to the girl's school Areishia Spirit Academy, which teaches noble young ladies how to make contracts with powerful spirits. On his way, he has the misfortune of catching sight of one of these lady spirit contractors bathing. She is Clair Rouge, and she is contracted to a spirit named Scarlet, though she has her sights set on a more powerful spirit. Kamito convinces her to lead him to the school, but she stops on the way to make a contract with this powerful spirit. When Kamito ends up 'stealing' the powerful spirit, it is revealed that he is actually an unheard-of male spirit contractor! What could be the origin of his power? Now Clair is determined that he's going to have to support her, since she believes that the spirit he now has power over what should rightly be hers.

(Source: MangaHelpers)