Ok, so it's not about the prom exactly, but the whole climactic scene of the movie takes place at the "big dance," so it counts. One of the longest running Broadway musicals of all time instantly became one of the most popular movie musicals ever, and it's still an insanely entertaining treat to watch. Despite being notorious for having one of the oldest looking casts of high-schoolers in existence, it's hard to think of anyone replacing John Travolta (back when he was so cute!) and Olivia Newton-John as the inimitable Danny and Sandy (not to mention Stockard Channing- who cares if she was 34 playing 17, for my money she's the best Rizzo ever). It's campy, fun and makes you want to jump up and sing along, even though you've seen it a million times. Grease is the still the word, baby!
Original 1978 trailer (I love how the announcer feels the need to repeat the actors' names about a hundred times, just in case you forget):