Whatever it states on your birth certificate is what the school go's by. Whatever they care to be outside school is up to them, and what they care to dress in. I don't see what the problem is with deciding on the length of a skirt, weather they be tall, short, tiny, plump or thin, its simple, knee length, slightly above or slightly below as always been the norm for a school skirt, after that its up to the teacher/ head to decide if anyone is breaking that rule. There could be many girls at that school who bought new skirts towards the end of last term, 2 or 3 in fact, so is the school going to bear that cost?
Bednobs, Lets just say a summer skirt is more hygienically suited for a girl, in your case you are happy to wear trousers, but not everyone is, and that is the problem at this school at present. Also it is a lot more comfortable for a girl sitting at a desk all day to wear a skirt.