I believe that the conventional wisdom is that skirts are the safer bet, but in practice either should be fine (unless you are applying to a very conservative school). The much more important thing is to look professional.
take my word and wear just a suit....nothing else....pants/skirts are details you are better off without them! first of all....it will show ur originality (for I don't think anyone has ever tried it)...it will show how brave u are (two risks...getting rejected...and getting arrested for indecent exposure)!
most important though.....don't forget to trim....ADCOMS don't like bushy applicant!