Excellent book recommendation. The girl thing is pretty, well, horrible. I looped with my kids for 6th and 7th grade and after 2 years together it was unrelenting. And some of the parents, moms especially, just had no idea that their precious daughters could be so mean. At least I was mean at home, too! I definitely was one of the mean girls, but I don't think anything I did in middle school could even come close to what I witnessed in the last 10 years or so. Probably the worst thing was hearing that one of my former students was beaten up by 3 other kids who were all on my team after they went to the high school.
But, usually it peaks by the beginning of 7th grade and as they get near 8th grade, they become more human...