Does sheer cover it?

In a BY type school, tznius is not taught as something you need just if someone wants to say kriyas shema in your presence. It is taught as THE standard of dress in the city, in the country, in the winter, in the summer, essentially, anywhere. Even in chedrey chedarim, one is not supposed to say that tznius does not apply, though just about all authorities permit women to don bathing suits to swim in a pool with only women around. I agree completely.
What of school rules? The flaw in the logic of the skirts past the knee with knee socks standard is that the school allows sheer stockings for both students and teachers. If one really holds that the lower part of the leg must be covered like sleeves cover the arms, then it doesn't make sense to allow a sheer covering. In truth, sheer stockings reveal far more and achieve a more alluring effect than the shortest of socks. Thus it seems that the rule of socks when sheer stockings are permitted is more an issue of conformity to a dress code than actual tznius. See the http://divreichaim.

Also see http://kallahmagazine.blogspot.com/2010/08/tznius-legacy-not-fit-for-translation.html