Does a woman have to wear stockings?
There is a dispute among authorities concerning the statement of the Gemara whereby the shok is considered an erva: Many maintain that this refers to the area beneath the knee, in which case stockings are obligatory for women (in the public domain), but others maintain that this refers to the area above the knee. (Mishna Berura 75 rules that it refers to the area above the knee; see Yabia Omer, Yoreh Deah 6,14; Divrei Yatziv Even Ha-Ezer 37, who cite further sources; most, however, write that it refers to the area below the knee. Note that the dispute refers primarily to the question of reciting berachos in front of women.).
Therefore, if the skirt reaches beneath the knee, a woman should act according to the custom in her place. If the custom is to wear stockings, as it is in many Orthodox communities, she should do so.
If, however, there is no custom to do so, or in a family environment (where there is no family custom to be stringent), it is permitted to rely on the lenient opinion and not wear stockings.