Well, this is part of the bigger question we've been tossing around, concerning the 'accuracy' of historical fiction for children. If something was relatively uncommon for the time, but evidence shows it wasn't unheard of - and the author gives at least a nod to indicate that it was uncommon - does it work or not?

Lady Gwyneth does state that she wants her two chamber-maids to learn how to read so they can read aloud to her, and teaching G. to read is all that involves a scrap of parchment - she's given a bit of slate and a lump of chalk to do the letters. So I don't know that it's impossible, despite the atypicality of it... (Nobody's taught numbers, at least.)