I think it's a good idea to have one skirt that all the girls have to wear. And schools are there to teach, not to police uniform. You may be able to persuade your dd to wear something reasonable, but there will be parents that don't/can't, and then you will have an even bigger problem on your hands.
Round here, the girls wear tiny, tight, jersy type skirts. And they are very short. It does not look good. And at the risk of being flamed for saying it, it honestly makes me think they look like some paedo's ideal wet dream.
They will find their own ways to 'express themselves', even when they all have to wear the same thing. They are teenagers, of course they will. I know we managed it at school and our uniform was very strict. Think of it as encouraging her to be more creative in the ways that she expresses herself. 