At the end of the Hand Jive dance scene, Marty, as well as others, slide on the gym floor. When she does, you can see on her one knee that she is wearing a kneepad.
I just watched a "Where Are They Now -- The Grease Girls" on VH1, and the actress that played Marty said that she had ABSOLUTELY no dancing talent, and she was very surprised that they chose her to be in the movie at all. So she was probably the only one wearing knee pads!
:-)
Well, i play gym court volley ball and when you slide on the floor with out a knee pad, your knees burn really bad so i don't blame her for wearing a knee pad