If you have daughters you have undoubtedly faced the question of how long a skirt must be to pass halachic muster. The answer to this important question can make or break a good shopping opportunity. The easy part to figure out is that since "shok b'isha erva" (Brachos 24a) a skirt has to cover the part of the leg called the "shok". The harder part is figuring our where on the leg the shok is.
The Torah tells us regarding a korban shelamim (VaYikra 7:32):
וְאֵת שׁוֹק הַיָּמִין תִּתְּנוּ תְרוּמָה לַכֹּהֵן מִזִּבְחֵי שַׁלְמֵיכֶם
Rashi comments: Shok refers to [the part of the animal’s hind leg extending] from the אַרְכּוּבָה [knee-joint, the bone and the flesh of which are usually] sold together with the head, up till the middle joint [of the upper leg] which is called רֶגֶל סֹבֶ שֶׁל. [The animal’s leg has three sections to it; thus, the שׁוֹק is the middle of those three sections.] In other words, shok is not the lower bone of the leg, but is the upper bone -- the thigh. The Pri Megadim, Mishna Berura, and Chazon Ish all conclude that a skirt must therefore cover at least from the knee and above, but below the knee is just a matter of minhag.
Things get tricky because Tosfos (Menachos 37) writes that human anatomy is different than animal anatomy and the shok on an animal does not correspond to what the shok is on a person. Tosfos writes that the shok is actually the lower part of the leg in people. The description of the term shok by the Bach (O.C. 75) as a place that if not for the chiddush din or ervah we would not consider prohibited to be exposed because it is normally covered in dirt seems to point to the lower foot as the area being discussed, not an area above the knee. Based on these sources some poskim (R' Vozner; the Chazon Ish also debates the proofs for this position) argue that a skirt should be much longer and cover far below the knee as well.
If the shiur of the Chazon Ish and Mishna Berura are good enough for you, your daughters may once in awhile find a skirt that passes muster in a regular store. If not, your daughters are pretty much limited to "frum" stores which cater to long skirt lengths.
In the school which my girls attend they are required to wear a uniform with a skirt that goes well below the knee, but they are also required to wear knee socks (socks which cover to the knee, not just the ankles) so that no part of the lower leg is exposed. My suspicion is that this is just a matter of conforming to what other similar schools set as their uniform rather than a formal psak. My impression is that the knee-sock rule is flaunted by most girls on weekends or outside school, but what really reinforces my belief that this is just a matter of uniform and not halacha is the fact that the school has Moros who wear sheer stockings or to cover their lower leg (below the knee). Obviously if you think a first grader needs to wear knee socks because the lower leg is ervah, then teachers who are religious role models should follow the same standard. (For the record, the Chazon Ish 16:8 writes that the age at which you start enforcing these rules depends on the maturity of the girl, so a first grader may have no halachic obligation to follow any of these rules to begin with.)