There’s something about the “new girl” in school that makes us wary.

The hair may be dyed blonde with streaks of pink, the shorts cut-off in that timeless teen “bad girl” fashion.

But she’s utterly unquestioning when Beaver (Daniel Webber) angles to copy off her English quiz paper. Her confidence goes well beyond any “I’ve figured out I’m a dish and have power over my male peers” years.

“I don’t DATE high school guys.”

And that casual way she handles a cigarette, that voice — smoky, sexy and wise.

“Where you from?” she’s asked.

“Which time?” is her too-clever come-back.

We’ve already seen the unfashionable corner of California where she’s moved, and the drugs. And we’ve picked up on Beaver’s peripheral connection to that, young, dopey and unsophisticated younger brother to Troy (Grant Harvey), both of them easily bullied by Wyatt (Pablo Schreiber),  a muscle-bound billboard for tattoos, intimidation and “cooking.”

When Wyatt hisses “I ain’t goin’ to jail for your mistakes,” Troy, one of his street dealers, takes it to heart. Beaver is new enough to this to be terrified.

And then, voila, “Kat” (Eliza Taylor) pops up, in school, at their beer busts, in their parties and when they’re not looking, in their business.

“Thumper” is a standard-issue police procedural of the “21 Jump Street” variety — cop young enough to pass for high school goes undercover to break up an epidemic in the making, a town where overdose deaths are making the news.

But writer-director Jordan Ross, of the MTV series “True Life,” maintains tension and fills in the fascinating back stories on these characters, peeking beyond drug abuse and arrest statistics, humanizing the entire genre eco-system.

These kids aren’t just “bad,” they have problems at home and narrowly focused options for escape, financial and personal. The villain may be a brute, but Schreiber’s Wyatt is introduced as a doting dad, on “suicide watch” for the toddlers he’s ill-equipped to have in his life. He has a point of view and a chip on his shoulder about how life put him here.

And Taylor, an Aussie TV and film (“The November Man”) actress, lets us see the distractions, pressures and lying on the fly necessary to pull one over on people who may not be that bright, but have the adrenaline of paranoia supercharging their suspicions. We may know pretty much where this is going. But she holds our interest through every interruption along the way.

All of the traps set for our heroine are tropes of the genre, the suspicion that even half-stoned teens have when she doesn’t smoke meth or shoot up with them, for instance. The only one that rings false is her attraction to Beaver.

Yes, the connection and empathy we buy into. The temptation? Nope. The kid’s a kid, and not that sharp at that.

But Ross, his leading lady and her terrifying quarry make “Thumper” a familiar yarn that’s engrossing, unnerving and very well told.

MPAA Rating: Unrated, with violence, teen drug abuse, sexual situations and profanity

Cast: Eliza Taylor, Pablo Schreiber, Daniel Webber, Lean Headey, Grant Harvey, Jazzy De Lisseer

Credits: Written and directed by Jordan Ross. An Orchard release.

Running time: 1:30