What kind of school do you go to? I didn’t think schools had cotillion’s anymore.
Ask your instructor if there is a dress code. Many cotillions require their members to wear a specific outfit, so ask before you show up.
The dresses used for fall/spring cotillion should be semi formal/formal. Light formal and a step above should be used, but nothing overly formal. It should be a reasonably “fun” event, with lightweight gowns dominating the dance floor. Don’t overdo it with something that won’t let you move freely during dancing…you need something that flows and moves with the body. It should be very modest – you’re showing that you’re a woman of substance and class, not that you’ve got a great rack or booty. Look in the department store’s prom dress section, or in stores like Anthropologie. The fall/winter cotillion consists of darker colors like that of red/black/navy, etc., while spring cotillion has lighter colors such as turquoise, pink, green, etc.
Wear dress shoes. Shoes are important because the cotillion participants will be on their feet for long periods of time. Shoes need to be comfortable. Wear the shoes the week before to limber them up and so your feet won’t ache all night.