At the beginning, when Jesse and Stephanie are practicing for picture day and Michelle comes in, Stephanie goes and sits in the coffee table. You can see that at 1:50, she is sitting there, at 2:04 she is gone, and at 2:09 she is there again.
Stephanie goes down just as the 'punching' prop (that breaks her nose) goes off. It is an obvious 'phoney' hit.