Hi! I have been dying to watch one movie... it's an old one, but the plot as I remember is that there are 2 girls. One are really famous and other is simple school girl, not popular, but she is designing clothes. And somehow those girls switched places, and that famous girl starts going to school, and other starts living famous life. And everything ends at the Prom :D so , maybe sombody can help me?

Sounds like Model Behavior (http://www.imdb.com/title/tt0218121/)