What should I wear to my yr 11 leavers embly?

My leavers embly is next week and I have no idea what to wear! I'm 15, 5ft 2, size 5-6 feet. I can wear high heels or dolly shoes as they don't fit my feet. I have wide toes but really tiny ankels which means shoes don't really fit me =S. So if anyone has any ideas please say!! Oh and the dress code is casual but formal.