Seeing as I am currently reading The Silver Chair right now, I noticed that Lewis throughout the whole book practicaly calls Eustace "Scrubb." and Jill "Jill" instead of "Pole."
Even when Eustace speaks he's Scrubb: "Dont be such a wet blanket." said Scrubb. but if Jill "Dont be such a wet blanket." said Jill.
I just wondered why Lewis never really wrote it by calling her Pole as much as he calls Eustace, Scrubb. Except, Puddle and Eustace call her Pole a lot I think.
I think that, historically, in boarding schools and a lot of private schools it was pretty common for male students to be referred to by their peers and by their teachers by their last names rather than their first names, so Eustace would probably have been called and thought of by his classmates as Scrubb unless they were his friends. Then they might have called him Eustace, as Jill eventually came to do, but at the beginning of SC, I definitely had the impression that Eustace didn't have a lot of friends and Jill certainly wasn't among them.
Lewis might have felt like this was a kind of masculine way to write about a girl, so he might have chosen to refer to Jill by her first name in the narration, but perhaps Puddlegum and Eustace, wanting to treat her as an equal (or "as one of the guys", you might say), called her Pole at first to mimic the way males who weren't close friends might address one another.
I don't necessarily think that it is meant to be a snub to just call someone, especially a male, by their last name. There are quite a few subcultures, like the athletic, where it is respectful and even affectionate to just call your friends or teammates by their last names. A lot of guys that I know are called by their last names or a nickname based off their last name by their friends, and they really like it. So, I think it is, in general, more customary for guys to just get called by their last name than it is for girls, and that might have been why Lewis was comfortable referring to Eustace as Scrubb but Jill as Jill.
I also think that the story was written more from Jill's perspective than from Eustace's, so she is more likely to think of herself as Jill and Eustace as Scrubb at least until she knows him better.
Those are just a couple of my theories. Hope they are interesting or helpful
