Seikoku no Ryuu Kishi

From Seven Seas:

Welcome to Ansullivan Dragonar Academy, a school with a difference: the students are dragon riders, each contracted with their own personal dragon. Ash Blake has the unique ability to ride anyone's dragon, but he's been waiting for what feels like forever for his own dragon to be born. He's thrilled when his dragon, Eco, finally appears--but she inexplicably takes the form of a beautiful human girl.

Eco's peculiar appearance is only the first unusual thing Ash faces. His life is about to become a whirlwind of spies, dragon races, and unexpected attention from both the proud Princess Sylvia and the beautiful president of the student council, Rebecca. All Ash wants is to acclimate Eco to the world, but someone out there wants his strange dragon on the dissecting table!