“Cry_Wolf” is one of those cheap thrillers about teenagers being scared of something that may or may not exist. It’s a pretty decent one, actually, as far as these things go, until its finale, which is either really clever or really stupid, depending on how on-board you are with the whole thing.

It’s set at an exclusive prep school called Westlake, in an unnamed town where a local girl was recently found murdered in the woods. Some of the Westlake kids — led by bad girl Dodger (Lindy Booth), herself a townie-turned-Westlaker — decide, for fun, to manufacture an urban legend about the killing. Through the magic of e-mail, they spread the word that a serial killer has terrorized several other prep schools in recent months, and that he always starts by killing a townie in the woods. So watch out, Westlake! His reign of blood is just beginning!

It’s not true, of course; no one knows who shot that townie, but the serial killer — they call him “The Wolf” — doesn’t exist. But the rest of the school doesn’t know that. And if there’s one thing teenagers believe, it’s everything they read on the Internet. Soon the school is abuzz with talk of The Wolf, and speculation over who he’ll kill next.

Here is where the film (written by Beau Bauman and Jeff Wadlow and directed by Wadlow) diverges from the path you think it will take. You’re assuming, as was I — and as the ads for the film suggest — that students are actually going to start dying, following the modus operandi that Dodger and friends made up for their fictional killer. And indeed, someone calling himself The Wolf does start sending instant messages to people online. Everyone in Dodger’s group of friends assumes it’s one of the others playing games. But what if it’s something else? What if the real murderer — someone really did kill that girl in the woods, after all — read the Wolf e-mail and decided to BECOME The Wolf?

It’s the speculation on that point that comprises the bulk of the film, with creepy things happening that are attributable either to friends playing a joke or to a real killer stalking the school, and we’re not sure which it is. It becomes less about killing and more about the power of gossip (reminding me of the movie “Gossip,” an equally enjoyable trashy thriller that has been forgotten by time).

Dodger and the new kid, Owen (Julian Morris), have a budding romance that is threatened by Dodger’s apparent affair with a teacher (played by Jon Bon Jovi). Owen’s roommate Tom (Jared Padalecki), a football type from Texas, blames Owen when their room is vandalized and Owen’s possessions are untouched. Randall (Jesse Janzen), a multiply-pierced pseudo-punk, disappears for a couple days and a piece of jewelry that was once attached to his lip shows up covered in blood. Mercedes (Sandra McCoy) and her boyfriend Lewis (Paul James) and their feminist friend Regina (Kristy Wu) round out the septet of friends, and any or all of them could be involved in the mysterious happenings.

Rest assured, there is eventually violence and death, and a fair amount of it. And then the twists come, in the classic one-two punch where No. 1 is the surprise ending and No. 2 is the surprise epilogue. Surprise No. 1 is nothing compared to No. 2.

As I said, your acceptance of these shockers depends largely on how “into” it you were to begin with. A few elements were absurd enough to make me laugh out loud. A few others seemed ludicrous — until the movie replayed brief snippets of itself to back up its story and I realized how much sense it actually made. I have a begrudging admiration for a movie that can set up a house of cards, wreck the house of cards, and then persuasively argue that the house wasn’t made of cards at all, but of impenetrable steel. “Cry_Wolf” is dumb, yeah, but it’s so convinced it’s smart that it almost makes you believe it, too.

C (1 hr., 30 min.; PG-13, some profanity, moderate blood and violence.)