It is called ‘Bang Bang You’re Dead‘ and I point blank refuse to say what it’s about because it is one of those stories that packs so much into a few short pages that it really deserves to be read.
Analysing it here would spoil it for anyone who wants to read it later.
I think Penguin UK reissued some stories by Spark recently, but I don’t think this one was included. It seems to be easier to get hold of the collection of the same title in the US. Apparently ‘Bang Bang You’re Dead’ was first published in 1961.
Since then the title has been used for a play, a couple of films and a song.