You can wear shorts under a skirt and know no one can see the lines through it .
The issue is not So much skirts vs trousers as the fact that the very skirts and trousers they Have to wear. Fitting kids at varying stages of development into the same 3 approved items and have them all feel and look smart and comfortable is an impossibility.
And even when people wear the approved items anyone who has a body shape that means it's tighter around the thighs or bottom or the hip waist ration means the skirt appears shorter or slips down etc means those kids are then sent home etc for wearring correct uniform.
Anyone woth an ounce of common sense knows the same item looks different on different people...
I have a big bum so skirts always look shorter on me.
I have a small waist and big hips and bum so any trousers that fit over the butt are massive around the waist and short legs mean they trail along the floor. Every pair of trousers I own is frayed at the bottom very badly.