My eldest 3 DDs have all been told off at some point about their skirts, but no matter what I say, the allure of fitting in with their friends is greater.
They wear school-issued kilts in winter (which, by the way, can most definitely still be rolled up). I actually don't mind these as much, as they are worn with thick tights and the pleats mean that they don't cling in the same way as straight skirts. I've caught all 3 older DDs with them rolled up on occasions. Although I'd rather they wore them at the knee or just above, the mid-thigh kilt doesn't make them look promiscuous or tarty. They just look like children with a vast tyre of fabric round their middles.
In the summer, the school loses its mind changes it's uniform. They wear plain straight skirts. These can be obtained from the school shop, but can also be got at most school uniform shops. I've noticed when dropping off that almost without exception, the girls' going into school have skirts that are a good couple of inches shorter in summer, and that's presumably before they get rolled up. This, twinned with socks and bare legs as opposed to tights, is more uncomfortable for me. I've had battles with the DDs about where we get the skirts from, how short they should be and how tight they should be. I cruelly have made them all wear skirts that end just above the kneecap. I'm bloody certain that all 3 of them roll them up - I've caught them a couple of times and read the riot act, and I've had the 'uniform slip' from school telling me their skirts are too short. Clearly, I am making them wear them longer, the school is telling them to wear them longer, yet they STILL find opportunities to wear them short.
I genuinely think they roll the skirts up because the other girls at school do it - it's pack instinct. Their friends all wear their skirts short, so they want to as well. I don't think it has much to do with being sexy or grown up - certainly for the younger 2. It's all about fitting in. DD4 who is still at primary school has eyed up shorter school skirts because her sisters wear them, and confidently tells me that when she's in year 7 she 'will get to wear higher skirts'.
I just tell myself that DD1 got great GCSES and is doing well at AS level. DD2 and DD3 are getting great reports and are happy. They're all kind hearted girls. If they flash their thighs to a bunch of their peers, I will get over it.