The Ol' Switcheroo

We've always said Hollywood is a lot like high school, just with hotter parties and no classes. And recently, Kristin Cavallari has been proving that adolescent behavior can extend past graduation. The former Laguna Beach star is obviously unhappy that her ex, Brody Jenner has moved on to Nicole Richie so quickly, and she's not afraid to show it. A while back, Kristin donned a shirt baring the slogan, "You Can Have Him" -- an obvious message to Nicole. Then, on Tuesday night, K.C. took her jealousy, er, rivalry even farther when she was all over DJ AM (aka Nicole's ex-fiancé), according to a Page Six source. Kristin "made sure everyone saw her arrive" with AM to a Guess party at Capitale, and proceeded to follow him to two other parties and flirted with him until 3 a.m. But taking a move from John Mayer, AM is putting out the fire before K.C. hits the tabs with tales of their romance (a la Jessica Simpson). "AM is dating -- but not [Kristin]," AM's rep told Page Six. "They are just friends." C'mon, Kristin -- you left Laguna behind, now leave the catty behavior there, too.