Most prom nights come with a theme. In the case of Clare and James, their high school had come up with a French-themed concept. The idea of the night was 'Twilight in Paris,' and so everyone had to dress like they were meeting at nighttime in the French capital.
It's a little strange for a high school prom theme - most of the students had never been to France. Clare and James didn't mind that. They bought into the theme, and put effort into their outfits.