I have 4 DDs and a constant battle with 3 of them. The school say 'knee length' skirts, no makeup, flat shoes...
DD1 (15) has a skirt which is probably 3 inches above her knee when she leaves the house. This is meant to be a compromise. She's also allowed mascara (she's very self conscious about her blond eyelashes). I've caught her on numerous occasions with her skirt rolled to buttock length, and the end of last term I caught her in a body-con lycra micro mini which she'd bought herself and changed into. She also has a tendency to sneak on two pounds of eyeliner. We've talked about it (what kind of impression she might give off, rules are for a reason etc.), I've grounded her ('I swear I'm not rolling it up any more mum'), I've confiscated the eyeliner...but in the long run, nothing works.
Honestly, if anyone has a suggestion, I'm up for it! (Trousers are not allowed at her school.)
DD2 (13) is better behaved than her sister in this respect. Her skirt is also around 3 inches above the knee. I've caught her rolling it up too, but not to the dizzy heights of her sister - hers is mid thigh at worst. We talked about keeping the rules etc, and she cried and said everyone does it. A quick swing past the school confirms this as the truth, so it's really difficult to police, and I'm not naive enough to think she's stopped rolling it up. Makeup wise, she sticks to mascara which I'm fine with tbh.
DD3 (11) has just started secondary. Day 1 she set off looking lovely in a knee length skirt. By the end of week 1, she came back with it rolled up to mid thigh. We had 'the talk', and now she's promised to wear it properly although has no doubt followed her sisters' example and just rolls it up when she leaves the house . She's already begging for makeup because 'everyone else wears it', but that's a definite no for now.
DD4 (8) is still at primary, and happily wears her skirt an inch or so above her knees. Long may it continue...
Long story short - some of us mums of girls are trying REALLY hard, but it's not easy. We're not bad mums, just exhausted ones...