Beyoncé Knowles surprised students at NYC's PS 161 yesterday when she appeared at a special flash dance session as part of First Lady Michelle Obama's "Let's Move" initiative to fight childhood obesity. The kids were rocking out to Beyoncé's just-recorded "Move Your Body," and they were plenty shocked to see the pop star appear in their midst! Beyoncé went casual for her day of fitness after a big Monday night out at the Costume Institute Gala. She rocked a Pucci gown that was so tight her husband, Jay-Z, had to help her up the stairs. Beyoncé spent the evening mingling with pals like a Stella McCartney-clad Gwyneth Paltrow, then the friends moved on to the official afterparty held at Crown.