depends on where you are going through recruitment. Some schools are "no frill" or have a pretty casual recruitment all together. Other schools are more formal. I would think that from what you're saying "formal" means like a cocktail dress...definitely NOT a prom dress. Also, at some schools, PNMs wear black dresses to pref. But, that wouldnt be required but it might mention it in the recruitment info. I would say dresses like these would probably work with what it sounds like they're suggesting.
just make sure its simple and classic. not too short or anything.