I agree with most people here that pants under the skirt is no problem. Pashuteh Yid’s klal godol in tznius: Adding material never lessens the level of tznius, and can only add to it, no matter where that material is added.
In mathematical terms:
The level of tznius is a monotonically increasing function of the amount of material.
BTW: A corollary of this has been discussed in other threads regarding slits. PY holds that a skirt which is say 40 inches long, but with a 5 inch slit is clearly more tzniusdig than a 35 inch long skirt. As before, since there is more material in the skirt with the slit, it is the more tzniusdig. Otherwise there must be some new issur of hosafas begadim which I was never previously aware of. When you add 5″ of material to a 35 inch long skirt, but don’t have enough to make it all the way around the skirt, you have violated an issur. You are better off not adding any material at all. Now tell me, does that make even one drop of sense? Hmmm, hosafas begadim.
I agree with Azi and Oomis, as I wrote at length before. The women’s job is to cover the halachic areas, and not to deliberately invite men to commit an immoral act. As long as she goes about her business, it is the man’s problem to control his yetzer hara, and not to invent all kinds of new prohibitions corresponding to whatever filth he has on his mind at that particular moment. He must simply grow up.