GAW- stockings and tights are for sure assur. This is brought down by Poskim.
I personally have always thought that pants were okay, but I never actually asked a sheilah, so it’s possible that I am wrong. My assumption (which may or may not be correct) was partially based on the fact that there were opinions at one time that it is muttar for girls to wear loose pants. L’maaseh, these opinions have not been accepted, and therefore it is assur today for girls to wear pants. But since loose pants may not be “technically assur”, I wouldn’t think it’s a problem for someone’s skirt to be above their knees if they are wearing pants.
I wouldn’t advise walking around with pants and a knee-length skirt, because when I see girls walking around like that, I think that it looks inappropriate – as though they are really wearing pants but stuck a skirt over it- and I think that some of these girls do end up removing the skirt eventually. However, I would think that if someone is going on a tiyul or bike riding or horseback riding, etc, and she is wearing pants under her skirt, it is okay if her skirts goes above her knees. But again, that is my own assumption, so I may be wrong.
Leggings are in-between, so I don’t know. I would think that they are similar to tights, but less problematic. Since Frum girls don’t wear leggings w/o a skirt, I can’t really see why this is any different.