The teacher specifically noted that the boys didn't have to wear dresses since, obviously, women wear and have worn articles of clothing that were not dresses. Unfortunately, as Franke recounts, instead of putting jeans and a t-shirt on her son, the mom "flipped out that her son might have to go to school in a dress and be humiliated by a project that required cross-dressing as part of an overarching 'gay agenda.'"
It's interesting how invested many people are in male and female costuming, albeit for different reasons. Whether a person is transgender or cisgender, that level of investment underscores how gendered clothing and gender presentation are key determinants to how others react to and treat us. As the "gender" of various articles and colors of clothing has changed throughout history and differs by culture, why articles of clothing are deemed "male" versus "female" (and vice versa) does not seem to be based in anything inherent about men or women. What, for instance, is it about a woman's anatomy that makes high heels "female"? What about the color pink?
What does seem to hold true is that many societies have used, and continue to use, clothing to mark gender, no matter what form the clothing actually takes. For instance, as Franke continues to recount, prior to World War I, pink was considered a color for boys while girls wore blue.
So what, then, is women's clothing?
The answer to that seems to be circular. Women's clothing is and always has been, simply, clothing that women wear. It is clothing that marks its wearer as a girl or woman which, in turn, informs others how they are to treat, respond to, and interact with us. It is often, you will notice, the most fundamentalist socieities- Muslim, Christian, Jewish- that demand strict adherence to gendered dress and grooming codes. These societies, not coincidentally, also make strong distinctions between the categories of male and female.
If you're looking for a more specific, less circular, answer to "what is women's clothing," Twisty offers:
"When I got a spam for 'men in wedding dresses' this morning I thought, hell yeah! I sure do wanna see some men in wedding dresses....
Men universally look asinine in women’s clothes, yeah? The reason for this, and for mild funniness in other low forms of humor, is incongruity. Nothing says 'I submit to my species’ disdain and surrender forthwith any claims to my own humanity' quite like a wedding dress. Women’s clothes are designed, according to a rigorous standard of misogyny, to communicate that the wearer is totally up for self-abasement. Men, on the other hand, are required by law not to be totally up for self-abasement. Therefore, in accordance with the laws of patriarchy, comedy and gender, a dude in a wedding dress is improbable and unnatural, thus causing the observer to laugh or retch or curl a cynical lip....
Women look natural in stupid clothes because women, as is stated in the Global Accords Governing Fair Use of Women, are biologically and culturally constrained to subsist as degraded masochists."
The above, coincidentally, is also the answer to another question. Why is it that high heels, make-up, and a long flowing dress are considered "beautiful" on a woman, but ridiculous, disgusting, and freakish on a man? What does the female costume truly mark?