We are talking primary school here, girls may be restricted in how they sit/ play, not talking about pervy adults, just they don't want the boys/ girls to tease them and don't want to show off their pants.
Sorry, I got confused because paddyann opened by saying her daughter, who must be an adult, had friends who were wearing them. (Assuming paddyann is a GM!)
However, M & S sell them up to age 16 and many 6th formers I know wear them too, so it isn't just primary age children. I just feel youngsters should feel free to wear what they like with no concerns either way.