I've found for these nights, it seems to be personal preference.
Being in university (and dressing up consisting of a clean pair of jeans) I go all out with the full length dress for formal etc.. but I'm sure a nice skirt and top would be acceptable as well. I've seen everything from people in jeans and t-shirts (who I am hoping were going to Topsiders...) to beautiful gowns.
Remember, your first priority is to have fun. If full gown is fun (like for me) then go for it, if you're more comfortable in a skirts, i'd say go for it as well.
Hope this helps
Katdb