Catholic School-girl skirts are pleated all the way around. Also, I've seen old paintings of traditional kilts that fall below the knee.

http://www.chicagohs.org/AOTM/Jan98/jan98fact3b.html

Somewhere around the knees, as opposed to halfway up the thighs or halfway down the calfs is probably a good indicator of kilt length. And we'll probably see reasonable exceptions to this "rule" as well.

A flat apron, pleats around the back, are necessary features for something to be called a "kilt".