I was the poster that said I do not care about the length of DD's skirt.
For the last 30 years, school girls have been wearing short skirts. DD is 12. Her school skirt is as long in comparison to her legs as it was all the way through primary school at 3, 5, 7, 9, 11. It is the same length as everyone else's. She isn't trying to be sexy now and she wasn't at 3. Anyone looking at the length of school girls' skirts and perceiving it as sexy, that is about them not the girls. People in general are not looking at teen boys in shorts doing PE and believing the length of the shorts is an attempt by the boys to be sexy.
As for coloured bra straps, it is the case that bra style tops (for girls who are still developing) and teen bras generally come in bright colours with pictures of birds, cats, beach scenes etc on them don't feel ready to leave childhood behind and move into 'adult' underwear. The same applies to a lot of brightly coloured pants for teen boys. I doubt anybody sees the waistband of DS's bright pants and thinks too sexy, should wear white to appear more demure, or indeed his vest, clearly visible through his school shirt.
A girl in a white bra and a school shirt would show far more of her physique. I used to work in an office where white shirts were required, and every time the sun shone through the windows, every nipple and bra seam was visible, unless the bra was padded, but of course girls wearing padded bras to hide the outline of their developing breast buds from public scrutiny is also considered too sexy by some.
As it happens, DD's main source of sexual harrasment is from an older teen boy who is a foot/tight fetishist who keeps taking her shoes off on the bus so he can rub her 'shiny tight toes.' I look forward to whatever bullshit advice posters have on what kind of footwear DD should choose to avoid other people judging her and touching her based on their perception of what message her start rite lace up brogues are giving others.