Sort of related to the 'singular he' discussion above - are most female teachers referred to as 'Ms.' in America? Is this a normal thing? I always liked the detail that most women had the honorific 'Ms.' in the books (if any was used), but maybe it actually is very commonly used there and wasn't a conscious choice.
(Actually, in particular I think there are a few times when the kids give each other snarky nicknames on the spot - like "Mr. Punctuality" - and the female versions always seemed to use Ms.)