I think a lot of it depends on the season and the area you live in, but in early fall in Chicago, I would probably look for a nice wrap-like dress and pumps. Dress it up with a great necklace and a bag, and you'll be fine. "Casual/Formal" is such a strange term, it can encompass too much. People will probably run the gamut from jeans to suits.
SO find something that makes you look GREAT, and that you feel confident in. Reunions can be so nerve-wracking, and if you haven't seen any of your classmates in a long time, you tend to compare in your head what they looked like then to what they look like now. (At least I do. Can't be just me!) So make sure you love what you're wearing, and that will translate to confidence.