When Mary suggests using her Christmas penny to buy a pencil for her and Laura to share, Laura says, "And you can own half of my penny!" If Laura had a penny also, why wouldn't she just buy her own? It couldn't have been very realistic for the two girls to switch back and forth with the same pencil during class, especially since they were different ages and therefore had different assignments to work on in school.
Reply: This is exactly the conversation the girls have in the book On the Banks of Plum Creek. Furthermore, later in this episode, the girls are shown each working on her own slate.