Of course girsl en masse deciding to roll their skirts up is not unrelated to the sexualisation of girls in society and what men like to look at etc.
The point though is that when an individual girl rolls her skirt up, this is not a definite signal that she wants 50 year old men to try and pick her up on the high street.
For the younger girls especially it's just "what you do". Why it's "what you do" is a good feminist question - because females showing their flesh is approved of in a patriarchal socieyt, but at the same time condemned, because if there's one thing girls and women need to do it's be kept walking that tightrope between "not enough" and "too much".
I was someone who wore what I wore without much of a thought about boys or men at all really - I wore things because I liked them, I rolle dmy skirt up because that was what you did, I chose things that I thought looked nice or good when I was in the 80s or the indie phase or whenever - to be part of that crowd with a nod to my own tastes I suppose. I am heteroseuxal so liked boys BUT I didn't think about "using" my body to lure them seuxally, I mean that didn't cross my mind. I don't think most schoolgirls think in those terms, do they? And yet, that is what men put on them, and adult women too a lot of the time. They put the brain of a woman who is like that - who does dress her body to be generally sexually appealing to the male eye - into the head of a 13 year old girl. Or any girl between puberty and whatever age. Some women think like that, do most? I don't know as I don't. I don't imagine they do. But I do know that it's incorrect to put assumed intentions of a fully mature, sexually active, experieneced woman who enjoys dressing to show her body off, onto a young girl. I think men do that because they like to think, no pretend, that it's true. Because then they feel justified in thinking the thoughts they thinking, or taking the actions they take.