Vampire Knight, Volume 2 (Vampire Knight)

Cross Academy is attended by two groups of students: the Day Class and the Night Class. At sunset, when the students of the Day Class return to their dorm, they cross paths with the Night Class on their way to school. Yuki Cross and Zero Kiryu are the Guardians of the school, there to protect the Day Class from the Academy's dark secret: the Night Class is full of vampires.