In "Lord of the Flies", how would the novel be different if it was about a group of girls? How would this change the novel's message?
Well, it's a counter-factual question: to change the boys into girls would entirely change the tone and shape of the novel. For one thing, there would be problems with the period: when Golding was writing the novel, feminism as such hadn't really started, and it would be, I think, unusual for an all-girls school to be flying their children out on a plane (a choir tour? Evacuated - without luggage?). You also wouldn't be able to have "the choir" for Jack to be in charge of: English public school classical choirs are, for some reason, nearly always all male - even to the present day (think of King's College, Cambridge, and St. Pauls... and so on). The choirboy is a bit of a British institution.
Golding also makes much of the stereotypical British "stiff upper lip", stoical gentleman. Here's Jack:
After all, we're not savages. We're English; and the English are best at everything. So we've got to do the right things.
That again, is specifically a male stereotype. With a female-driven novel, that would also have to change.
So in short it would be a very different novel. That said, I don't think it would necessarily have to alter Golding's central theme: "the darkness of man's heart", and the propensity of humans for savage, evil behaviour. It is, I think, a universal theme that Golding chooses to explicate through a group of boys. It could, I think - though in a very different way - be explained through a group of girls.