If they're dressy shorts, you'll have to wear a dressy top and dressy flats with them or they'll look weird, and that might make your ensemble TOO dressy. It looks more like the girls in the pictures are wearing twill or chino material skirts. I would just go with that.
Sidenote: LOL at "please do not wear athletic shorts on bid day." I mean at the fact that they have to state it.