“Short Term 12” is the kind of teen-oriented movie that’s often dismissed as an “after-school special,” but thanks to a nimble young cast it’s frequently better than that.

A familiar, emotional drama about an institution for at-risk teens, it sometimes plays like an actors workshop. Not that that’s a bad thing, especially when it’s handled by an ensemble this talented.

Brie Larson, best-known for playing Toni Collette’s rebel daughter in Showtime’s “The United States of Tara,” has the starring role: Grace, the 20-something manager of the place. Full of secrets that are gradually and sometimes painstakingly revealed, her character drives the spare narrative.

John Gallagher Jr., who had key roles in HBO’s “The Newsroom” and Kenneth Lonergan’s “Margaret” (he was the shy student who couldn’t quite ask Anna Paquin for a date), is her devoted boyfriend, Mason. He wants to marry her but feels exasperated because he knows she has been less than forthcoming with him about her past.

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When she slaps and bloodies him during sex, he’s genuinely shocked and disappointed. In the film’s most troubling scene, he bravely breaks up with her; he sees no future in their relationship. Both actors bring a startling intensity to this moment, though it’s Gallagher who gets the most revealing lines.

The movie is an expansion of writer-director Destin Daniel Cretton’s short with the same name, which won a Sundance prize in 2008 and dealt with a more tentative relationship. If the feature film feels compact in some scenes and lumpy in others, the added length could be the reason.

John Hartl: johnhartl@yahoo.com