Lilly in the Middle (Hardcover, Turtleback School & Library ed.)

Lilly tells her classmates that their teacher looks like a wolf -- and they believe her! Lilly and her friends have a substitute teacher named Mr. Wulf, who is taking them on an overnight field trip. Lilly jokes that he looks like a werewolf and is surprised to find that her friends take her seriously. On the bus, her friends won't sit with her. When they have to go on a moonlit walk, no one wants to go because they are afraid of Mr. Wulf. They say Lilly has told them he is a werewolf.