She notes changes in his behavior: He does not want to wear corduroy overalls anymore, no longer waves goodbye to her, slams the door when he comes home, and speaks insolently to his father.
During lunchtime conversations, Laurie begins telling his parents about an ill-behaved boy in his class named Charles, who frequently misbehaves.
On some days, Laurie arrives home late and says that the teacher kept the whole class after school as a result of Charles' behavior.
By the end of the week, however, Charles has reverted to his old self when he makes a girl in his class say a profane word to the teacher.
[1] Ruth Franklin, in Shirley Jackson: A Rather Haunted Life, notes that an alternative interpretation is that Charles is a supernatural being that only Laurie can see.