Seriously mfm, instead of saying there is no reason, you should ask someone who had a reason. It’s arrogant to assume that because you’re not aware of a reason that there can’t be one.
I don’t think there are any good reasons. Looking back on high school, some of my favorite acquaintances were girls and I wouldn’t have meant them if I went to a boys’ school. I get why boarding schools are segregated, because of their dormitories, but that’s just a logistics reason.