At my kids' (secondary) school the girls ARE allowed to wear trousers in theory, but only the official school trousers, which cost £25 and are unbelievably hideous. The uniform supplier doesn't even keep them in stock as noone buys them. I've literally only once seen a pair. It's quite sneaky, as they can say that the girls are allowed to wear trousers but 'noone chooses to'. The boys can, of course, wear any style of trousers in the right colour, from any supplier they like.
They have recently stopped enforcing the 'only the right trousers' rule - dd went to school in plain trousers a few times, and only got compliments and people saying, 'Those look smart. Are you allowed to wear them?' But of course parents aren't buying trousers because they think they're not allowed.
The girls uniform and boys' uniform are even different COLOURS because... those are the colours the uniforms were when the girls and boys schools combined... FIFTY YEARS AGO.
It means you the only thing you can hand down between a boy and a girl is the tie.
Apart from the stupid trouser rule, it's clearly discriminatory because the boys can wear any trousers in the right colour and the girls have to wear the official school skirt, meaning the girls uniform is considerably more expensive. I've held back from making a fuss about it on the basis that the school has had enough to deal with this year. But dd says that 'Gender neutral school uniform please - we'd like to just pick from a list of items' is requested at every school council meeting and just gets ignored.