I headed to work in one of my long skirts the other day.
I hadn't ridden in it before, but it worked just fine. It is an A-line cut with a slit on the front left side. This allowed my left knee to poke up out of the skirt while pedaling. But it was long enough that nothing was revealed... I did push the fabric further down between my legs in front, just to be modest.
The hem stayed neatly away from chains and spokes. But when I got to work, I discovered the very corner of the hem, next to the slit, had rubbed against the wheel and gotten a few smudges on it. They came out pretty easily in the bathroom, unlike the nasty grease stain I got on an escalator last year! That one has never come out; you can still faintly see it in the top photo.
To prevent any further incidents, I secured the errant bit of hem with a coordinated hair clip. This kept the fabric tucked neatly beside my left knee and unsoiled.