I lived in a country for a while that made it illegal for women to wear trousers or shorts.
That's policing women's clothes.
Literally.
A school announcing ' this is the school polo shirt, this is the school jumper, and these are the choices of shorts or trousers for all' is not policing girls.
I haven't met a teenage girl that possesses not a single pair or trousers/jeans/shorts/sweatpants/ in her wardrobe. Girls already wear trousers and they are not a 'boys' garment.
This isn't imposing an outlandish requirement on girls. They're school trousers, not dayglo onesies.
Kids get to wear what they want outside schools, but schools have a duty of care to create an environment that doesn't foster inequalities. If the practical effect of including skirts in a uniform choice is that only one sex 'chooses' them, and then either self-censors their own behaviour (how you must sit nicely, how physically active you can be, how short you can make them) or is policed by peers, then you aren't fostering that healthy environment.
As I said, recognising that people have the wrong motivations for banning skirts shouldn't result in throwing the baby out with the bathwater.
Stilettos are banned too - a dodgy headmaster suggesting the reason is because they are too sexy and distracting for boys, rather than an impractical and damaging item that disproportionately hurts only one sex, shouldn't result in a backlash of protests for the right to wear stilettos.
Where skirts DO remain on uniform lists, they should of course be available for boys. But I won't mourn some schools dropping them entirely. I think girls will benefit overall from this.