What should I wear to Sadie Hawkins?

I'm going to have a Sadie Hawkins dance next week at my school and I'm not exactly sure what to wear. Our dance has no specific theme and it's semi-formal. I was just wondering if I should wear something a bit simple like a blouse and a skirt with ballet flats or should I wear something a little fancier such as a dress. The dance is semi-formal so I'm not sure if a dress would be too much but I'm not sure if a blouse and skirt would be too simple. Please help!