Why do you think Laurie reported Charles's good behavior grimly, while he had delighted in telling about Charles's bad behavior?
In the ironic ending to Shirley Jackson's short story "Charles" Laurie's mother discovers that although her son has been faithfully reporting the deeds and misdeeds of a boy named Charles in his kindergarten class there is in reality no Charles. Laurie is really Charles, which should come as no surprise to the reader in light of Laurie's behavior at home where he torments his baby sister and often talks back to his father. Charles is made up because Laurie doesn't care to receive punishment at home as well as school and because he seems to be beyond reproach in the eyes of his mother. Laurie seems to relish his bad behavior at school. He gleefully reports of Charles's bad conduct. He shouts out that Charles yelled in school or hit another child.
Eventually, the kindergarten teacher appears to have made progress with Laurie's behavior, much to the child's dismay. His usual happy reports are labeled grim and he shrugs apathetically when he tells his mother that Charles was the teacher's helper and was rewarded with an apple. Obviously the malevolent manifestation of Charles is more pleasing to Laurie. When Charles/Laurie reverts to the negative, Laurie reports to his mother with "a voice slightly awed." For Laurie, the life of the miscreant is much more exciting than abiding by the rules of his teacher and the school.