A Vampire in the Neighborhood
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A Vampire in the Neighborhood is a short story by R. L. Stine where a group of friends believe that the new girl in school is actually a vampire waiting to attack them.
The new girl is called Helga Nuegenstorm and she looks really strange and behaves oddly (e.g., she had black ringlets of hair, wears old fashioned clothes, pale skin and has gray eyes). Thus she is the perfect candidate for the monster they believe she was.
The narrator tries to test her by inviting Helga to sit down with her friends, but she replies: "I don't eat." This confirms their suspicions and they decide to investigate more information about her... So they go to her house and look at the bedroom window.
In the end it turns out Helga isn't a vampire, but as the expected Goosebumps twist, the other kids are!