When I was growing up there was a clear divide between boys and girls. Being the only girl in a large family, I enjoyed growing up with boys and playing their games, with their toys, etc. Then we entered grade school and learned that boys and girls were too different to play together and I, being the only girl, was ostracized from the only playmates I had.
I remember girls in my classes calling boys gross, boys calling girls dumb, and the divide was encouraged, if not enforced, by our parents and teachers. We were told that interacting with each other was inappropriate. As a girl, I was told that boys were "out to get me" and I better watch my back because befriending them would bring nothing but trouble. I dunno what the boys were told but, from the ridiculous crap I've heard, it wasn't anything good.
I would look beyond this particular group of girls that you work with. Is this an attitude brought into the group by one or two girls with particularly strong opinions and personalities? Are the parents friends? Do they belong to the same community? Schools? Churches? Book clubs? Is this an attitude the parents are collectively instilling in their children?