My 18 year old never, ever wears a dress. The smartest she ever gets is black skinnies/cigarette trews and a tux-esque jacket.
Her friends tend to wear skirts and tops. Last weekend one of them looked stunning in a fitted black off the shoulder top and a full midi skirt - v 50s inspired but non fancy dress-esque. Another was wearing a short tea dress type thing, with little flowers.