Sandy from "Grease"
Sandy's transformation at the end of "Grease" is arguably way cooler than her good-girl look that she wears in the rest of the movie. It's her final attempt to make Danny realize that she's cool enough to run with the fast crowd, and you'd better believe it worked.
Claire from "The Breakfast Club"
Molly Ringwald's Claire was the typecast rich girl in the high school movie trope. She was outspoken and narcissistic, but by the end of the film, she falls for Bender and realizes it wouldn't hurt to loosen up a little and stop caring about what other people think of her.