If you remember that moral of the story is like the lesson or principle the author wishes to convey, think about what you learned from the story. With each new story about the antics of another boy in the class named "Charles", the reader finds humor both in the parents' reaction to the naughty boy stories and in the stories themselves. When at the end, the parents find out that there is no boy named "Charles" in their son's class, and that it is their own son creating the havoc described, they must face the truth that Laurie is the culprit. Being a parent myself, I believe that Jackson is making us look at childhood as a learning time for parents and child, that mistakes made can be fixed or learned from, that nothing is so great that it cannot be dealt with even if the parents are unaware at first. I also think that Jackson is making a gentle statement about the difficulty of parenting and raising the child you want him to be, especially if you remain unaware or unquestioning with your child.
Because you are younger than I am, you may find another moral in the story which fits as well as mine. Look carefully at what principle you learned from this story.