Thank you.This is not logical, I realize, but actually we might very well say "a slit down the side," too. It's all a question of how the speaker perceives the slit, not how the slit was formed.
But what if for some reason your eye started at her blouse (maybe it's really cute or a really flamboyant pattern or something). Isn't it possible that you'd then say "down"? I would.I don't usually disagree with Kate, but in this case, I do. If the slit starts at the hem (what you call the "low edge"), I'd definite use only up, not "down".
...It's all a question of how the speaker perceives the slit, not how the slit was formed.
I seeIf I am pointing out how much leg it being shown with the slit, then I would say it is "slit up" (or "slit up to her thigh")
If I am pointing out that the pencil skirt allows freedom of leg motion to a greater degree that other pencil skirts then I might say it is "slit down".