"A Vampire in the Neighborhood" is the tenth and final short story in the book Tales to Give You Goosebumps.
A group of middle schoolers is convinced that the new student, Helga, is a vampire. After all, she wears all black, is deathly pale, she even wears black lipstick. They decide they will spy on her to determine if she really is a vampire or not.
Over the course of a few nights, the children sneak out to her house, trying to see if she casts a reflection in the household mirror, sleeps in a coffin, and so fourth until finally Helga grows tired of all the stalking and she confronts them. They ask if she is a vampire, to which she sighs and says she is. They then ask her to show them her fangs, which she jokingly replies that she wants to see their own.
They proceed to do so, and Helga is horrified, considering she was just kidding. But they were not. They are real vampires and they had wished she was too. This results in them attacking her and turning her into a vampire too.