theworriedwell
*tell them that they need to cover their knickers with shorts in case boys/passing men/teachers get turned on by being reminded of what is underneath the knickers.* That is being incredibly rude and judgemental about boys and men. I don't think it is about anyone being turned on by being reminded of what is underneath the knickers it is more about people not wanting to see what is supposed to be underneath the knickers.
I can't think of an occasion when a schoolaged boy has been exposed because of the design of his clothes (many years volunteering in primary schools) and I think girls should get the same dignity so let them wear trousers/shorts/leggings or whatever but let them have the same level of cover as boys get.
So why do people 'not want to see what is under the knickers', if not because these parts of the body have been sexualised?
Otherwise the rest of your post is pretty much exactly what I was arguing for.