How important is it to wear a suit to an interview?

(for an actual full-time job in higher ed). Keep in mind it’s Chicago and like 80 degrees outside. And I don’t own a suit. 

Can I get away with a nice business-y tank dress (black or grey tweed) and a cardigan? Do I have to wear nylons/tights or is it apropos to be bare legged? The dress goes just past my kneecaps.

I know nothing.