Rosario + Vampire

Due to poor grades, Tsukune Aono, an average high school student, has been unable to get accepted into any private academy of his choice. By exceptional accident, the only school to accept him turns out to be anything but ordinary; the school is attended by youkai disguised as humans. Fearing for his life, he attempts to escape only to meet Moka Akashiya, a beautiful girl who just happens to be a vampire. Despite this, Tsukune decides to stay at the academy, hiding his human nature from students and teachers. Tsukune also discovers that when the rosary around Moka's neck is removed, her true nature emerges.

Synopsis from Anime News Network