Any rules that exclude the wearing of shorts, are just rubbish.
It's SO simple, why make it hard on yerself! "All pupils should wear at least: trousers, shorts or skirts, of neutral colour, and short or long sleeve shirts. Ties should be worn unless staff say otherwise." Genderless, objective (no useless statements like "Unless it's hot") and probably even enforceable (To be fair, I've never had to).
If a pupil chooses to wear shorts when it's -5, fine - let 'em shiver - they won't do it again. If a boy wants to wear a skirt, so what? (He'll probably get Fucking Hell from his mates, but hey...).
When I were a lad, I HAD to wear shorts. All year round - it was Teh Roolz. Aged 7, I had to walk the three miles home from school, in my shorts, in a blizzard - when all the buses were cancelled. It was fucking cold. I survived.