I wonder what would have happened if more of the established characters had stayed behind in Britain and we got glimpses into both schools. A few do, like the Dawbarns, but then they go to Switzerland later on.
I think part of the reason why it feels frozen in time is because the Platz is so cut off from society. In Tyrol, Armishire and even St Briavels (because of the mainland connection, the Christy family and Kester Bellever), there are local people and families who give it a sense of community: the herdsmen, local clergy, the apple seller, Herr Braun, the Temple clan, various shopkeepers and Welsh servants with weird names, the local sweet shop owner and so on. In Switzerland, they're out in bumfuck nowhere with this little community of expats, Old Girls, doctors and their families, and people like the Rutherfords who've got family at the Sanatorium. There's very few Swiss families or local people: OK, there's the families they stay with on the obligatory half-term trips but they're all in different areas and they're not part of the Platz community. And that's one reason why the setting just doesn't work. They're not as close to a town as they were in the other settings, it's no wonder they were still putting their hair up and using exactly the same expressions as they were in the '20s and '30s ('alarums and excursions', 'one so young and fair', 'my only aunt Sophonisba', various Scottish expressions from non-Scottish characters, and the classic 'work like a n***'). Occasionally you'll get new slang like 'fab' or 'smashing' but the girls' vocabulary is stuck in the past. I'm guessing it was because EBD was pretty old by then and not interacting with girls like she was earlier on.