I have an idea well 2 ideas with a similar theme.
One girl is in the shower or sleeping (naked or in underwear take your pick). The other two girls rush in and frantically scream that there's a fire and that she need sto leave. The other girl runs downstairs in a towel or bedsheet with the other girls following her out. The minute she gets outside the other girls quickly snap away her bedsheet/towel and lock her out. The other girl screams drawing a crowd (or not, but she is locked out).
The second one involves school. Now find a way for one girl to become naked in a main part of the school near the entrance or outside. She should preferably be tied up. Now when this happens she should think that because class is in session for the next 45 minutes she should be fine. Then someone pulls the fire alarm drawing THE ENTIRE SCHOOL out to where she is (You could make her bubbles and have her cry if you were really feeling cruel.)
Oh and I really do like the embarrassing stories page someone mentioned earlier pity it hasn't been updated in a long time.
Perhaps we can draw inspiration from some of the stories, like this one (one of my favorites)
http://www.thatsembarrassing.com/story-1092.htm
or this one
http://www.thatsembarrassing.com/story/1126
or this one
http://www.thatsembarrassing.com/story-977.htm
(man reading those stories really makes you feel like a sadist sometimes)