Practical question...

...for female and/or fashion-savvy readers: what does one wear to teach a class when it is ten zillion degrees outside, and twenty zillion on the subway platform? I don't need to dress office-formal, but informal summer clothing has a way of making the wearer look either skanky, ten years old, or, worse yet, some combination. I've tried variations of the t-shirt dress (longer on me than on the 6' model), but these strike me--and more importantly the skeevy men of the subway--as a bit... revealing. Is the answer discomfort? Accepting that summer teaching has a different dress code than winter teaching? Bringing a change of clothes for once I get to the air-conditioned building? Abandoning my current look for a combination of linen and hemp-derived fabrics that send the message that I am in my mid-sixties and will not let my friends vote Republican? I am seriously leaning towards this last option, which is clarifying, all of a sudden, how it came to be that teachers dress in that distinctly teacher-y way.