Scarlett Johansson was already a star name by the time her prom night rolled around. She appeared in 'Lost in Translation' when she was only 17 years old, so everybody knew that she was going to be a big deal in the world of cinema. Johansson didn't need to dress to impress anybody on her big night, so she went for the 'too cool for school' dressed-down look.
Dressing down when everyone around you is dressing up is a big risk to take, because if you get it wrong, you're going to stick out like a sore thumb. She had one big advantage though - she's Scarlett Johansson. She could have turned up on the night wearing a potato sack, and she'd still have looked great. She's managed to make herself look effortlessly cool, and as an added bonus she's also dressed perfectly to hit the clubs as soon as prom night kicks out. No need to bring a back-up outfit!