Avril Lavigne is an adorable 17-year-old tomboy from Ontario. She is short and girlishly skinny with a thin, pretty face — she could pass for Jennifer Aniston’s little sister — and long, straight hair. Unlike teen pop stars of the past several years, she dresses down. She wears baggy pants with a wallet chain and a ratty T-shirt, and her signature fashion statement is a man’s tie that looks as if it probably came out of her father’s closet. She wasn’t wearing one last Saturday night at Mix 98.5’s MixFest concert at the FleetCenter, but gaggles of teens in the audience had already adopted the look.

Lavigne was backed by a band of young boys who dressed and talked like the rude boys of Blink-182 or Sum 41 (in fact, she shares management with her fellow Canadians in Sum, and until recently her bass player was an original Sum 41 member). In the video for her winsome hit "Complicated," skateboarders work the half-pipe in the background, and the follow-up single is called "Sk8er Boi," where the hero is a superstar skateboarder who becomes a rock star (he’s rejected by a ballet dancer, but Lavigne snaps him up and then delivers the kind of keep-your-hands-off-my-man putdown that’s become a staple of black-pop ballads).

The teen-pop revolution required the music industry to engage in a massive build-up of creative weaponry — songwriters, producers, arrangers. Now, as the pop boom begins to bust, the labels are faced with the task of beating their Britney-clone swords into rock-and-roll plowshares. Gone are the synthesizers and drum machines and choreographed dance routines, and in their place is Warped Tour–style skate-punk chic. But the songs remain curiously similar. Lavigne is credited as a co-writer on all of Let Go’s songs, but she had lots of help. Clif Magness, who helmed a great deal of the disc, is an old partner of Alanis Morissette songwriter Glen Ballard; the LA production team the Matrix, who co-wrote and produced both "Complicated" and "Ska8er Boi," have written for Christina Aguilera, Ricky Martin, and Liz Phair.

At the FleetCenter, Avril opened with a moderately revved-up "Sk8er Boi" — her punchiest number — and then segued into "Nobody’s Fool," a folkie fable of empowerment with a few rap verses. Her rapping was too cute to be terrible. It was like watching Alicia Silverstone freestyle: her cluelessness was a built-in charm. "Losing Grip" has a soaring rock chorus out of the Incubus songbook; the song’s verses sound like sheets left over from Shakira’s Laundry Service. But the rest of the set languished in mid-tempo acoustic doldrums. For all Lavigne’s skaterish, Punky Brewster spunk, even "Complicated" was curiously sedated.