I found a website
http://www.fast-rewind.com/ that has all kinds of info on Footloose. (If the link takes you to their main page just find footloose and click....)
Seems they reshot that last dance scene entirely after they finished the movie so maybe SJP couldn't get back to film it or they just didn't edit her in.
Interesting stuff there...including that Lori Singer was 27! This was one of those movies that the actors were in their late 20's but you would never know it.. not like Grease where you knew John Travolta wasn't 17!