With her pooch in tow, the Gossip Girl star celebrates her 21st b-day on the New York City set of her show. Plus: Ashlee Simpson, Lindsay Lohan, Paris Hilton and more

PEP SQUAD

On hand to show their high school spirit, Ashley Tisdale and Zac Efron share a hug before getting pumped up at Disney's High School Musical 3: Senior Year Ultimate Fan Pep Rally at the El Capitan Theatre in Hollywood on Saturday.