Something like certain standards for height, weight, BMI, and fat/muscle ratio.
A healthy girl, to stay healthy, needs a certain amount of body fat, because adipose tissue is a major endocrine organ, producing hormones such as leptin and estrogen. If a female's % of fat goes down too far, she starts getting some of the same kind of symptoms as menopause, e.g. not only a cessation of ovulation and menses, but also demineralization of the bones, which would make things VERY risky for a girl in a contact sport.
Any sports physiologists out there? What should the metrics be?
If we were talking strictly about recreational sports, it seems to me it would be reasonable to have co-ed teams based on size rather than on age/grade. My brother and I played on the same soccer team in a league at a Navy base one year, and he was two years older than I. (11 and 13, maybe?) Soccer isn’t the same as football, of course, but everyone’s the same size as *someone*, if you get what I mean.
However, school sports, in many areas, are big business. It’s all about winning, not about the good of the participants, no matter what sex they are.
This is in my neck of the woods and we are a fairly strict conservative area. The parents are nuts for wanting their daughter to play football - it is dangerous enough for boys. Yeah, I know - the libs cry equality and all that - but for those dunderheads - men and women are built differently and their bodies prove that.