Essentially, the message of the story is that the creation of a personal identity is a difficult and complicated process, regardless of age. This is shown clearly through the character of Laurie, a young child who begins kindergarten. Jackson argues that this is a very important time for Laurie because it affects his development as an individual. She makes this clear in the first paragraph when she describes the physical changes he undergoes: he starts to swagger, for example, and no longer waves goodbye to his mother.
Laurie's creation of an alter-ego called Charles comes as a great surprise to his mother and, in fact, provides the final plot twist. But it also shows that there are important mental and emotional changes in Laurie's development which mirror the physical changes that he has experienced. For the first time, he is testing boundaries, as shown by his various antics in class, and learning that his actions have consequences, as demonstrated by his various punishments.
That Jackson offers no explanation for Laurie's actions suggests that she believes them to be a natural part of growing up. For her, forming a sense of identity is a complicated process which parents and teachers just have to accept.