It Was His Sled: While admittedly not hard to figure out, even without having watched any of the film adaptations — the book doesn't make it explicitly clear until the end that Annabel's mother was, in fact, possessing her body the whole time. Annabel's mother did her best to act like Annabel during the switch.
There are plenty of other cringeworthy incidents that are difficult to read from a modern point of view: various expectations of women being expected to do all of the cooking and housework, Ape Face laughing at Boris because he can cook, the principal smoking in the school, Annabel's father talking down to her mother like a child, and the school having no idea how to deal with a student like Annabel who doesn't learn well through traditional teaching methods.
