Perhaps it starts with a strange book in the school library. A curious student finds an old, dusty book that tells the story of vampires. As they read it, they unknowingly summon a vampire into the high school world. This vampire then starts to interact with the students, causing all sorts of chaos and mystery in the hallways and classrooms.
Another path could be a curse placed on the school long ago. There was an ancient wrong done at the school site, and as a result, a vampire is now bound to the high school. A group of students start to notice strange happenings, like missing pets and unexplained noises at night. They begin to investigate and eventually discover the vampire lurking in the old, abandoned part of the school building.