The answer to this question can be found in Chapter Four. Mr. Morrison, after picking up the kids, comes up with a rather novel way of making them take responsibility for their disobedience. When he tells the kids that he is not going to tell their mother about the way in which they broke her rules, he then goes on to qualify his statement, saying that he has decided not to tell her, "'Cause I'm leaving it up to you to tell her." Disbelievingly, Cassie watches as Stacey agrees to this and says that he will tell her himself, taking responsibility for his own mistakes rather than being a child and having to have others tell his mother of his various misdemeanours. What is interesting is the way that this text describes what happens between Stacey and Mr. Morrison after Stacey makes this resolution:
But he seemed not to hear us as his eyes met Mr. Morrison's and the two of them smiled in subtle understanding, the distance between them fading.
The "distance between them" fades because Stacey, by assuming responsibility for his actions, is becoming a man, which is something that Cassie is not yet old enough to appreciate.