Oh my goodness, can you just imagine a boycott of Joey's touching Christmas play, or the sale of work? Chalet Girls Fight Back? The Chalet School on Strike? I know the recent fill-in deals with the aftermath of the Marilyn Evans affair, so I suppose that's the closest we can get to the details of what exactly went wrong.
(Btw, to me, prioritising one's own schoolwork over Head Girl duties seems totally reasonable. But the school I went to was miles apart from the CS and had no such thing as prefects etc. Would it really be normal to expect a schoolgirl to put those duties over her own educational achievement? ISTR something somewhere - poss also post-Marilyn, or poss wrt having Mary-Lou 'fix' Ted when that's Joey's big idea but the heads are reluctant? - about not lumping too much responsibility on people who have exams that term, or something?)
The staff also play cards, and I think some of the other girls do too. It's only bad if you're putting money on it (like Diana) or if it's Sunday (like Elma), I think. Doesn't one of the staff get mildly bankrupted by the others, on some occasion? I'm inclined to think it's Ruth Derwent in one of the first Swiss books.
I think the bit about Elma is deliberately vague. I tend to read it as the "you're going to end up barefoot and pregnant, and worse, EXPELLED" scene, and I don't think EBD would have been unaware of that reading - but I think it's deliberate that you could also more innocently take it as a fear that Elma would permanently ruin her reputation simply by having unchaperoned coffee with a man. She also doesn't ever say what exactly the letters say, do they? There seems to be some arrangement to meet, but whether that's for a sedate cup of coffee in Zurich or for a dirty weekend in Paris is never made clear. It has to just sound like The Worst Thing Possible, doesn't it?
Then there's the bit where they go to the opera and hear Gretchen am Spinnraden and Casanova(??) too, in case you missed the moral first time round. 
I really quite like Oberland. But I suppose it must be tricky to juggle writing books about 18yos with having an audience of mainly much younger girls.
My DGM also watches those antiques programmes and believes her bits of beloved tat ornaments and oddments are worth lots of money too. Again, often comparing hand-crafted china with mass-produced plastic stuff. I just nod - it's not like she's got any intention of selling - but my DM can't resist arguing the point...