Suppose the plot of the book had a lot of girls or boys with girls. Do you think the same violent and bad things would have happened on the island?
I ask my students this question every year in class. Every year it's the same answer. There wouldn't be as much physical violence, but the verbal and emotional abuse would be just as bad. The teenage girls in my class are very open about talking about how a Piggy type personality would be completely alienated and shut off from the group on day one; a Jack type personality would be called all kinds of "wonderful" names, and a Ralph type personality would be considered a "goody-goody". In all, the girls in my class suggest that nobody would be nice to anybody on this island.
To answer this question with evidence, it would be good to view the film "Heathers" or "Mean Girls," both of which dispel the notion that girls are kinder, sweeter people than boys. Another reference would be the book Queen Bees and Wannabes by Rosalind Wiseman. This book explains ways in which adolescent girls form hierarchies of power to include some and exclude others, and the cruelty they might employ to accomplish this.