Some of Yuu's classmates are shown to have dragon "partners"/"suitors" (to avoid a more heavy term), and the plot runs around avoiding having the dragon meet the designated girl.
I am talking about when a dragon comes for a girl and her Ryuumon glows.
Examples are Iris and Leviathan, Tear and Basilisk, the girl Mitsuki killed (she turned into a dragon in the backstory).
But the school has dozens of D girls. Are there that much dragon beasts rampaging around?