An important theme in the story "Charles" is people's blindness to the flaws of those closest to them—and also their enabling of these flaws.
Although Laurie is constantly talking about Charles's antics at school, it never once occurs to his mother, the narrator of the story, that "Charles" could be Laurie. Laurie tells his parents that Charles gets spanked at school for being fresh and also for hitting the teacher. Meanwhile, he is also doing disrespectful things at home as he is telling these stories. His parents simply let him get away with this behavior and don't associate it with Charles. For example, after his mother asks him a question twice at the dinner table and Laurie ignores her, the parents do not stop him from leaving:
Laurie slid off his chair, took a cookie, and left, while his father was still saying, “See here, young man.”
Laurie continues to tell his parents about Charles's misdeeds, and the parents continue to be blind to Laurie's faults. For example, he tells a rude joke to his father and gets away with it.
Rather than recognizing that Laurie is crying out for the boundaries and discipline that his teacher is finally providing for him, the mother worries that school may be "unsettling" him.
By the time of the parent-teacher conference, Laurie appears to be responding well to the constraints of the classroom, learning to control himself, helping hand out supplies, and getting the approval of his teacher—all under the guise of being "Charles."
At the end of the story, it must be a shock to the mother to realize that Charles is Laurie and to confront her own misconceptions about her son.