I don't normally post on this part of the board, but I lurk
and felt I wanted to chime in here.
At your daughter's age, she may be getting "attention" from peers about her looks, especially boys which is making her uncomfortable with her body. Maybe the root of the problem is that someone said something to her or implied something regarding her body or specifically her legs? It could even be that she read somewhere that legs are sexy to men, and that scared her? Does she wear loose / long sleeved tops to cover her chest and arms? I just really think there might be more to this than just simple pre-teen power struggle, but then again, I may be wrong and it is nothing more that that. I would try sitting down and talking to her, letting her know that she is welcome to wear pants if she chooses to, but that you are there for her to talk to when she wants to.
All I know is that when I was a teen, I wore anything short because I liked my legs and the attention it got me. (Drove my poor parents crazy
: )As I got older I realized I didn't need that KIND of attention and stopped wearing short stuff. I almost never wear shorts anymore, even at the beach I wear capri cargos until I go in the water. Very rarely, shile at home, I'll wear shorts, but even they are longer now, almost to my knees.