Why are girls in my high school saying they are vampires?

They sit right behind me and right next to me in almost every class I have and they are always asking if they can bite my neck and they are serious about it. I feel uncomfortable when people ask me that so what should I do. I am actually serious this is getting weird and they are not flirting because they do this to everyone and we all want to hide in a field of garlic. Should I just let them get over this phase over time or what?