Depends on where you live I guess. One of my friends at Uni grew up in quite a posh area and went to an all-boys grammar school, but I'm pretty sure mixed-sex is the norm. I mean my first ever friend in primary school was a girl, it's just that all the ones after that were boys and I just stuck with male friends after that. Doesn't help that I started getting crushes on the girls in class when puberty hit so it was even harder to approach them normally