At DSs school boys only wear trousers in Yr5 and Yr6 but are allowed to still wear shorts if they want. None of them do though because trousers are seen as a badge of seniority/maturity.
However, somewhat odly DS1 encouraged the boys chose to wear skorts rather than shorts in a recent house netball match so they had the same kit as the girls. Girls have to wear skorts but no rule boys can't. This from a boy who won't be seen dead wearing pink because its 'girly'.
I do admire the lad in this case and it looks like the school will review its policy.