From Rock! Rock! Rock! in 1956:
http://www.youtube.com/watch?v=RCt4_Dwt-Lk

If you blink, you'll miss her. She is seen near the end of the film dancing. And probably the best shot is at 1:22:03 applauding.

Woah, she was young there. From 1957 according to the video, meaning she was 18 then. Though according to IMDB it confirms the movie is from '56, making her 17 there.