At the start, there was an incident where a normal kid found out about the bite marks and speculated vamps. Yuki suspected the prez to be but didn't think it would be true.
Then the vamp prez who got struck by the star scales( basically aphrodisiac-like for vamps) bit Yuki by accident. Yuki being calm-headed asks about vamps and the prez explains. The prez is depressed tho as he never wanted to show this side to Yuki. But Yuki was okay about it.