This is one of the more interesting Chalet School books. This was first published in 1956, and it's explicitly about class. It's inherently interesting, because this tends to be a series that doesn't question its own premise.
It's explicitly about class in a pretty tangled way. Rosamund is offered a scholarship to this posh boarding school and doesn't want to go; her parents point out it will offer her great opportunities in life, plus they don't really like the friends she has in their current neighborhood; Joan (one friend) mocks her because Rosamund's mother was a lady's maid while Joan's mother worked in a shop, a step up; Rosamund worries what people will say about her parents' occupations at school, and it turns out that they don't care at all - though Joan thinks she'll be able to hold the "your father is a gardener" bit over Rosamund's head without people caring that her own mother worked in a shop -
I can't quite sort out the class distinctions in this world, and I'm left with the feeling that the author can't, either. This is not helped by Rosamund discovering twice, a few dozen pages apart, that she's expected to be trilingual during lessons.
And then we have to the Chalet School morality angle, where Joan comes in wearing makeup (GASP), the world's most quickly applied nail polish, and a dress that's too mature for her, looking "cheaply pretty" - that's a direct quote - and it reads as more class-based judgement, from this school that ostensibly doesn't care who your parents are - as long as you make the same decisions as them, I guess? And don't stand out? Or talk about boys? That conversation where Mary-Lou talks to Dr. Jack about Joan's "cheap ideas" (hmmmm, consistent adjective, that) - it's a good conversation, from a character standpoint! Mary-Lou worries about sounding like a snob, and she and her friends don't talk about boys, and then Jack says that Joan probably belongs to "the sort of people who finish school at fifteen" and are much more grown-up about that sort of thing than Chalet School girls.
(This might be the first book where Jack gets to speak actual paragraphs since Jo to the Rescue. That's, by my count, eight kids ago.)
And so despite the fact that the conversation ends with Jack telling Mary-Lou to help Joan and be a friend, this ends up being about class in more telling way, I think, than it was meant to be.
The times, they are a-changin' - but the Chalet School doesn't seem to notice.