08/12/2008 at 01:10 PM EDT
She may play a reformed wild girl on Gossip Girl
– but Blake Lively
says her romantic life isn't nearly as scandalous as her onscreen persona, Serena van der Woodsen.
"There wasn't a period in my life like Serena went through, where I was doing drugs and having sex with my best friend's boyfriend," Lively, 20, tells Cosmopolitan
in its September issue. "I never went through that kind of crazy."
In fact, the actress is steadily dating
costar Penn Badgley.
"I've kissed just three people in my life, other than stuff that I've done for TV or movies," she says. "I know – I'm weird!
"I hope Brad Pitt
doesn't hear this," Lively says, teasing. "He's never going to want to marry me!"
While her character attends New York prep school and leads a champagne-fueled life, Lively tells the magazine her high school experience was far more down to earth.
"I was class president, on the cheerleading squad, in a competitive show choir, and in, like, six different clubs," she says.
"I don't drink. I don't do drugs," she says, deadpanning, "But I do sleep around a lot."