I assume you are referring to the Shirley Jackson short story "Charles." In the story, the mother sadly comments that her "sweet-voiced" little boy Laurie is growing up. He begins misbehaving at home and then returns from school with stories about a classmate named Charles and his wild antics in the classroom. Nearly every day Laurie tells about a new predicament that Charles has gotten himself into. The mother begins to wonder if Charles' influence is the cause of Laurie's own bad behavior.
When Laurie's mother attends a PTA meeting several weeks later, she questions the teacher about Charles. To the mother's surprise, she discovers that there is no boy named Charles in Laurie's class. The implication is that it is Laurie himself who has been causing all the trouble at school.
The underlying theme? Parents, don't believe everything your child tells you.