I find it amusing when I read any comment on the length of the skirt for schoolgirls. Do know it is now customary and normal for any schools that have skirts for these to be rolled up, no matter how much the skirt ma not be suitable.
When I went to Grammar school back in the early 1950's we it was the time of the fashion for very long skirts (New Look). We had to kneel down at the start of every term and had our gymslip length measured with a ruler to ensure that it was at least 3 inches above the knee. That made them quite short (fine for those of some of my friends with lovely thin long legs - but for me - short, fat legs it was a nightmare) So, it seems as if when fashion is long length skirts, schools dictate short skirts and visa versa.
I do tend to think that anyone who can see anything slightest sexual in whatever length a school skirt is has their own problems.
To think that London, or anywhere else for that matter, does not belong to any one demographic