I would go to the cocktail/evening section of a dept store (or online) and search for cocktail pants. A dressy pair of cocktail (sometimes categorized as "holiday") would work for this situation. There are also dressy, chiffon blouses that are paired with these pants. The look is definitely cocktail and dressy, yet fits in with your requirements. Any other type of pant, imo, is going to look too casual.
This outfit will also work for your sorority formal or any other type occasion where a cocktail dress is required, so it may be worth it to splurge and get a really good pair of pants.
Top:
http://shop.nordstrom.com/S/2994888/...bo=6005555&P=2
Pants:
http://shop.nordstrom.com/S/2911162?refsid=237331&refcat=0%7e2376776%7e2374327 %7e2374333%7e2374362&SourceID=1&SlotID=2&origin=co ordinating
