Writer-director Megan Park zeros in on a female student’s struggle to find her footing after surviving a campus massacre in this film.

Only three adults appear onscreen, their roles key but decidedly supporting in the teen-centric drama, which foregrounds a quintet of superb young actors, led by Jenna Ortega.

The California suburb where Park has set her debut feature is multicultural and privileged. Kids don’t want for comfort, though some of them make do with absentee parents. Sixteen-year-old Vada (Ortega) doesn’t fall into the latter category; her folks are, as she tells a friend, “good parents.” She’s a sharp-witted high schooler who basically falls out of bed in the morning, barely brushing her hair, in contrast to her carefully dressed up whip-smart younger sister, Amelia (Lumi Pollack). They are close despite their difference. When Amelia finds herself in need, the person she texts is her big sister.

But soon Vada goes through something that upends every aspect of her life and creates distance in her closest relationships. Shots ring out in her school, and for six terrifying minutes she cowers in a bathroom stall with two students she knows only by sight: the glamorous Mia (Maddie Ziegler) and Quinton (Niles Fitch), who stumbles into the girls’ bathroom covered in blood, having just witnessed his brother being shot.

The sight of Quinton a few scenes later, greeting Vada and Mia at his brother’s funeral, is heartbreaking. He’s a lanky teen in a grown-up’s suit. Fitch’s performance conveys depths in Quinton’s silences as well as between the conversational lines.

Vada in particular excels at playing chill and deflecting. We feel the sting of resentment when she watches her mother and sister being silly in the kitchen; that sort of spontaneity is now foreign to her. Vada’s defense mechanisms give way to something openhearted as she and Mia are drawn toward each other. Via text and video chats and then IRL, at the gated property where Mia essentially lives alone, they hold the world at bay, putting off their return to school as long as their parents let them.

For Mia, that means indefinitely. Her fathers, both artists, are away in Japan — and apparently her traumatic experience isn’t enough to bring even one of them back home. As Vada blurts out during their awkward getting-to-know-you phase, Mia is much gentler than the image she projects in the dance videos she posts. Vada might also be figuring out that the girl she’s long considered out of her league with her 82,000 Instagram followers is friendless and lonely.

Vada’s parents — her anxious mother (Julie Bowen) and Latino dad-of-few-words (John Ortiz) — vacillate between hyper-attentive hovering and stepping back to give her space. It takes a surprisingly long time, in terms of the movie’s running time, before they send her to therapy.

“The Fallout,” fortunately, is not a therapy film. And precisely because Park is not interested in the orthodoxy of therapy-speak, the two psychotherapy sessions she includes are especially potent, with a strong and steadying Shailene Woodley defying movie-psychologist stereotypes. ”The Fallout” doesn’t pretend there are easy answers for Vada. Her withdrawal enrages her good friend Nick, who becomes an antigun activist.

After the shooting, Vada gradually gathers a collection of memorial programs and places these tributes to her murdered classmates in a box that she keeps in her bedroom. It’s a spacious room adorned with fairy lights. As glib as these smart teens can be, they’re not quite out of childhood.

(SD-Agencies)