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.

I live in western nc. The reunion is this coming Saturday

Wrap dress and pumps would probably work there too, I think! Personally, I can't do wraps because their waist is too high and far away from my natural one, but many of the faux-wraps can work for me.

Have a great time! Let us know what you decide to wear!

Thank you for your help

Crystal, what is your body type? Would you be comfortable posting a picture of yourself in jeans and a simple top? (You can hide your face). Also, can you tell us more about what your style is like, e.g. whether to tend to dress classic, trendy, sporty etc. It'll help us make more suggestions, based on what would be flattering to your specific shape and "fashion persona".

I'm a special ed. teacher get to wear jeans most of the time.Small frame, height is 5'2" weight 131 lbs. 7 1/2 in. from bust to hips. Love to dress up when the time is called for. We have been told event is casual/formal. Will be held at 6:00pm at Country Club. Don't want to under dress or over dress.