You should also consider where you are going to be shopping. Many stores particularly independent boutiques will keep a record of the dresses they have sold to people in each school. Some stores will not sell the same style to the same school. However, if you buy your dress from a high street store then you could see someone with the same dress at your school formal.
You should also reconsider buying a dress from an online foreign site. Many of these sites will not fully disclose the import charges for their dresses. There have also been a number of horror stories about people who have bought their dress online only to get a dress that looks nothing like the picture and not having any recourse.