I'm a mother of boys, so thank God I don't have experience of trying to moderate teenage girls' clothing.
However, I have been a teenage girl, in the 1980s, when my role models were Siouxsie Sioux (probably spelt wrongly, sorry), Claire Grogan, Debbie Harry, Annie Lennox.
Can you imagine any of those women walking around with their butt cheeks and cracks on show (and Debbie Harry was/is one of the sexiest women ever)?
Yes, it's an old-fashioned view, but my friends and I would never have been allowed to set foot outside our homes with our arses on show.
At school we wore massive thick, dark coloured knickers over our "ordinary" knickers when playing hockey or lacrosse or netball or tennis, to cover up (very frankly) any hint of bum-crack or cunt/labia when our short skirts flew/blew up during exercise. We viewed it, grumpily, as repression, at the time, but actually it was a freedom, to be athletic.
I disagree strongly with people saying it's fine for girls to dress this way, as it's just the current fashion, it's "empowering". No. It's demeaning. It's disempowering.
Those teenage boys shambling around with their jeans or "trackies" pulled down under their Calvins (or whatever is the current fashion in idiot underpants) are laughable, right? Why encourage girls to go that low?