This scene was funny and at the same time embarrassing. If you look closely at the frame by frame (just keep pressing pause and play until you pause it at the start of the cheerleading list) you'll see that Justin wasn't supposed to have Martha as the partner. This was obvious from his and Amber's reactions. The girl he was supposed to have was a bit shorter than him and much thinner. The jock was envious of Amber's affections for him, so he wanted to give him the toughest challenge a.k.a enormous girl. When Justin tries to lift her, she falls on him with his face stuck in her butt. This is even worse than imaginable when she mentions to him that she isn't wearing panties. Probably the most embarrassing scene for Justin in the movie.