We didn't have cotillion in England, but I really enjoyed everything about it. One of the most fun parts was finding just the right dress. It was also quite stressful because the dress guidelines we were given were very strict.
Here are some photos:
If you are looking for a white cotillion or graduation dress, here are some suggestions:
1 * 2 * 3
My advice, look for something modest and timeless. Keep the accessories minimal, small diamond or pearl earrings would be perfect. And take lots of pictures.