Here is the plot I got from wikipedia:
Vampire Knight mostly takes place in Cross Academy, a prestigious private school with an unusual class structure. The student body is divided into two classes: the Day Class, which is made up of ordinary students, and the Night Class, an elite group of 'beautiful people.' Unbeknownst to the majority of students, and even most of the staff, the Night Class is made up entirely of vampires. The heroine, Yuki Cross, is one of the few humans who knows the school's secret. She serves as a Guardian, dedicated to maintaining the peaceful coexistence of the Day and Night Classes with her childhood friend Zero, who lost his parents to vampires at an early age. As the story goes on, the plot moves away from keeping the Night Class' secret from the Day Class and into Yuki and Kaname's past as Asato Ichijo (Takuma Ichijou's grandfather and a member of the Vampire Council), Shizuka Hio, Rido Kuran (Kaname's Uncle), and Ichiru Kiryu (Zero's long lost twin brother), make their way to the academy.
