Let me clear up several misconceptions that you apparently have with Shirley Jackson's short story, "Charles."
First, Laurie is not a girl; he is a little boy, and the fact that he is hung with a name usually reserved for girls could well be a part of his behavioral problem. Secondly (and I hope I don't ruin the ending, since it appears you haven't read it yet), Laurie and Charles turn out to be one and the same person.
Laurie is just beginning to attend first grade. His mother, who describes him as a "sweet-voiced tot" at the beginning of the story, watches him transform into an insolently "raucous... swaggering character." He picks up lots of bad habits quickly, and his mother assumes that many of these come from the new friend he has at school--the class bad-boy, Charles. Charles gets in trouble on a regular basis at school, and Laurie has a new story nearly every day concerning his friend's exploits. When Laurie's mom visits school and asks about Charles, the teacher tells her that there is no Charles in her class. It is only then that the mother--and the reader--discovers the truth: Laurie is the troublemaker, but he assumes the nom de plume of Charles when relating the stories to his parents.
Charles/Laurie is transformed into a rude, sassy brat right before his mother's--and the teacher's--eyes. Charles hits--and is then spanked by--the teacher. He hits a girl in class, causing her to bleed; kick's "the teacher's friend;" throws chalk; stamps his feet; curses; and is "fresh." His behavior at home also changes for the worse, and he speaks disrespectfully to both of his parents regularly. The father seems even more clueless than the mother, and he barely seems aware of the changes, even when he laughs "insanely."
Is Laurie displaying signs of a behavioral disorder? Is he showing overtly masculine behavior to overcompensate for his name? Why does he find the need to create this imaginary persona? These are things to consider as you read (or reread) the story.