The effects are decent, the acting desultory in “Prey for the Devil,” a School for Exorcists thriller that promises more than it delivers.

Consider the setting of Robert Zappia’s script, based on a story by Todd R. Jones and Earl Richey Jones. It’s the St. Michael the Archangel School for Exorcists in Boston.

I don’t know about you, but the minute I saw that, I thought “HOGWARTS for Exorcists? GENIUS!”

Alas, that may have been the idea, as this picture — like EVERY horror movie — is treated as a potential franchise. But the colorless students inside this “school,” the formulaic way exorcist stories are populated (there’s always a young victim, always a body count trying to “exorcise” her) play out and the generic CCTV, one-way-mirror “training” rooms, the panic buttons in every patient’s room tech in a hospital decorated by scores and scores of candles turn this tale into an exercise in tedium.

Jacqueline Byers of Showtime’s “Roadies” stars as Sister Ann, a nun/nurse at St. Michael’s, and a survivor of trauma herself. We see her childhood in the film’s opening scenes, chased, tormented and abused with a hair comb by her psychotic mother.

Sister Ann knew better. Mom wasn’t schizophrenic or “just” schizophrenic. That “voice inside her head” had to be…the DEVIL.

That had everything to do with Sister Ann’s choice of vocation, she lets the in house shrink (Virginia Madsen) at St. Michael’s know. But she’s not just here to be a nurse. She ducks into the all-male-priest classes of Father Quinn (Colin Salmon) to hear how the Mother Church is “losing a war that has been raging for centuries.”

Sister Ann would love to become female Exorcist Ann, although all the academics there know the Catholic Church had one 800 years ago — St. Catherine of Siena. And as Sister Ann comforts a child in their care (Posy Taylor), we see her point. Nobody else can talk to Natalie inside whatever is making her skitter up the walls like a victim of “The Ring.”

But as Sister Ann tries to put herself into the game, sneaking off with Father Dante (Christian Navarro) to try and treat his pregnant, possessed sister (best effects in the movie), diving into Natalie’s case, she runs up against the patriarchy in the form of the cadaverous Monsignor (the late Ben Cross) in charge.

Byers is a fresh-faced and freckled Canadian blonde who is perfectly credible as a nun with a purpose. But she doesn’t give us much to grab hold of in this character. With rare exceptions, she and most everybody in “Prey” underreacts to the impossible, terrifying and imperiling things they see. If you’re not that scared, why should we be?

Go back to “The Exorcist.” Even the grizzled veteran of the rite, played by the great Max von Sydow, flinches at what he’s witnessing. His younger apprentice (Jason Miller) may not want to let us see his fear, but he can’t hide it.

If the idea here is that everybody in this school has seen it all, do something with that. Failing that, you’ve got to let us see people, save for the most hardened, freaked-out by what’s going on. Just yanking characters and hurling them against walls or out of the frame down a dark and deadly corridor in the “catacombs” below the hospital (of course) isn’t enough.

Nor is hewing so close to “Exorcist” formula that we know the story beats before they drop.

Rating: PG-13 for violent and disturbing content, terror, thematic elements and brief language.

Cast: Jacqueline Byers, Posy Taylor, Colin Salmon, Christan Navarro, Ben Cross and Virginia Madsen.

Credits: Directed by Daniel Stamm, scripted by Robert Zappia. A Lionsgate release.

Running time: 1:33