Whose words reveal that Charles is Laurie?

In this story by Shirley Jackson about a horrid boy named Charles who is supposedly in the narrator's son Laurie's kindergarten class, we discover that Charles is Laurie when the narrator (Laurie's mother) speaks to his teacher.
Laurie has been coming home telling his parents that Charles is very naughty and is repeatedly punished for his misdemeanors. It is clear that Laurie enjoys talking about Charles's audacity, and he has a different story to tell every day. Laurie's parents become very interested in meeting Charles's mother. His mother hopes that she will see her at the PTA meeting, but, unfortunately, she doesn't and ends up speaking to Laurie's teacher.
When Laurie's mother inquires about her son's behavior, it becomes apparent that the Charles he has been speaking about is himself. The teacher states:

“We had a little trouble adjusting, the first week or so, but now he’s a fine little helper. With occasional lapses, of course.”

Laurie has also been saying that Charles has become a good little helper and that he has not been in trouble for some time.
When Laurie's mother comments that her son's attitude must be because of Charles's influence, the teacher replies that they don't have any Charles in the kindergarten. Her statement confirms the fact that Laurie is, in fact, Charles.