Do we ever see a new girl not being shocked and horrified by the languages? 
It's such a funny one, because EBD is so clearly trying to make an Important Point about snobbery being wrong. There are places which are really understanding - like when ML's all "Jack! She talks- about- boys!", and he says but she comes from people where it's normal to leave school and be a grownup with a job at fifteen, it's different. But then there's Miss Annersley saying "it's hard for you because you've found your standards are lower than ours"!
In the end, poor Joan's 'sin' is taken to be pride, isn't it? And pride is something EBD/the CS is as keen to stamp on in the 'well bred' brigade just as much.
Funny moment when it's acknowledged that Joey oughtn't to join in with the Joan problem when Nancy brings her back from wherever-it-was-she-ran-away-to and Hilda gets to deal with it on her own! Also funny when Len runs Rosamund over to find out from Joey why Mary-Lou is so special, I agree.
I like Joan. I like how, even when she is still cheerfully hating the school, she's strategic enough to think she ought to get all the language learning she can out of it, at any rate, for her employment prospects. She's canny, isn't she, and EBD clearly explicitly recognises that and I think sees it as a good thing just as much as one borne of circumstances.
There is a lot of bonkers 'advice-seeking', though. When the heads go to consult the staff-room about accepting Joan and the agreed suggestion is 'write and ask for more information' - srsly?