The overall tsnius issue is not about the inches or the hem, but what makes a girl look like a bas melech. When a girl wants to purchase a skirt, she should be taught to think before she spends whether or not this item is going to give her an appearance that befits a refined girl. Would she be comfortable being seen in it before her menahel or menahelles? Tznius is a hard test for the girls who fight the pervasive culture which teaches them to be the opposite of refined and modest. It is a real struggle for even some of the finest girls. In the Torah, gemara and mishna there are few examples of tznius because up until WW1 most women were tznius. In Europe a non Jewish woman did not walk on the street in pants or with her head uncovered! There were no short skirts or slits. An ankle was considered a pritzus thing to reveal! So in the last 100 years there has been an unprecedented amount of decline in the overall morality of human beings, of values and of modesty. Does a Jew just go with the flow of society and blend in with the goy? Why would you want to? Remember that one reason we were saved in the first geula from Mitzrayim was because we did not change our dress. We stood out from the surrounding culture and did not follow their “styles”. Why has that changed today?