Maybe Laurie thought it is fun to fool his parents so that he could continue to make up stories every day.
Clearly Laurie is living a double life. He is a terror at school and revels in telling about the exploits of Charles when he gets home. He kind of has the best of both worlds. At school he gets to act out, and at home he gets to act out as he retells and re-lives all the naughty things Charles says and does without getting in trouble. For example, he gets to kind of tell his dad off as he describes Charles--something he would never get by with as Laurie. Why he feels the need to do all this is another matter altogether.
Laurie wants to make his parents think that he's a nice little boy comparing to "Charles"
Laurie wants to attract attention from his parents and his parents didn't put any effort in correcting Laurie.