The adorable klutz of Disney’s popular TV series takes her bumbling adolescent angst to the big screen. The plot is rudimentary: after an awful embarrassment — a disastrous junior-high graduation that gets internationally telecast, thanks to her little brother — Lizzie heads off on a class trip to Rome, where she’s mistaken for a pop diva and perpetuates the charade to stay in the good graces of a hunky Italian crooner (Yani Gellman). It’s the ultimate schoolgirl fantasy, but Lizzie’s ball-busting chaperone (Alex Borstein) looms at every turn and threatens to burst the bubble. This could be Hilary Duff’s swan song as a tweener princess: the wholesome nymphette already has a Top 10 MTV video, and here she hits the Colosseum in a swanky outfit and performs a vacuous pop number. Her wide-eyed effervescence and gleaming smile have appeal, but the stammering wears thin. Director Jim Fall offsets the bubblegum perkiness by interspersing æsthetically framed portraits of Rome’s timeless architecture. (90 minutes)