@ignatiusjreilly
It's in The Chalet School in the Oberland, when they go to a concert in Interlaken, shortly before the train incident. It doesn't mention anything about her being a widow in my copy, but it's the Armada version so all the juicy bits have been removed.
Thank you,
@ignatiusjreilly! I've never owned a copy of that and I don't think I've read it since I was in my teens (which is not today or yesterday). And yes, I think the uncut version has some other audience member telling them about her being widowed and going back to her singing career.
It still amuses me, though, because I think implicitly, it's not a matter of her age making the song inappropriate (after all, we've seen the CS girls regularly act characters much older than themselves in various plays and pantomimes), it's the fact that it's a passionate and very sexy lied, sung by a fallen woman -- I think the implicit message is that you should only be singing about kisses and longing if you're married and hence have a legiimate reason for having kissed a man. In which case you shouldn't be on the stage, you should be at home darning your husband's socks, or whatever it is Miss Annersley says about marriage at some point. But in that case, Bill, Mlle and Miss Norton, all unmarried, should definitely not be listening to it and pronouncing authoritatively on it, either. 
Do you suppose Joey and Verity's voice training, such as it was, had to explicitly avoid operatic arias by fallen women? Joey's repertoire, insofar as we ever hear about it seems to be largely English art songs or folk, doesn't it?