Shakespeare knew, or maybe came up with, the first rule of teenager-in-love tales. There must be parental disapproval of Montague-vs.-Capulet proportions, or some other frictional force beyond too-tight jeans, to keep them from dissolving into mush. Because it lacks such tension, “If I Stay” just barely makes the B honor roll, despite a fine job by dark-indie darling Chloë Grace Moretz in her mainstream breakout performance.

With the complicated roles she already has under her youthful belt — including a brutal, forlorn vampire in “Let Me In” and pint-size assassin Hit-Girl in the “Kick-Ass” franchise — 17-year-old Moretz is practically slumming it to portray Mia, a good-girl cellist who dreams of going to Juilliard. Relative unknown Jamie Blackley (who is Manx, by the way, like those tailless cats from the Isle of Man) is Adam, frontman for an up-and-coming Portland rock band popular enough to attract groupies wanting him to sign a boob. But Mia’s parents don’t object to him. They used to be him.

Dad gave up his gig as drummer for the Nasty Bruises to become a teacher when Mia’s younger brother Teddy came along, and now the mighty tight family unit leads a cheery, middle-class boho existence — until a devastating car accident on a snowy road sends them all to intensive care, with Mia in a coma.

As in the bestselling YA novel by Gayle Forman on which it is based, “If I Stay” flits between flashbacks of Mia’s family life and romance with Adam and the hospital, where her invisible spirit self, clad in an ethereally adorable sweater/skirt combo, has split off from the prone motionless body hooked up to tubes and monitors and can only pad wistfully about, observing her grieving loved ones and hearing more bad news.

For its appealing leads and touching story line, the movie may strike a chord with some starry-eyed kids, but not the hippest ones. The “Smithereens” of its generation, it ain’t. True fans of both the classical and punk/alt-rock genres will cringe at songs that seem more like the Christmas-gift picks of an out-of-touch uncle than in-the-know peers.

Adam name-drops the Ramones and dons a Sonic Youth T-shirt, but that can’t stop him from looking and sounding like a cross between John Mayer and Harry Styles when he opens his mouth and sweetly strums that guitar. Mia regularly trots out the Prelude from Bach’s Cello Suite No. 1, a hackneyed Hollywood favorite, and has stickers inside her locker proclaiming that she hearts Yo-Yo Ma.

The supporting cast is serviceable, with a couple of standouts. As ex-rocker-chick mom Kat Hall, Mireille Enos, who till now has seemed destined to play pasty-faced glumsters (“The Killing,” “Big Love,” “World War Z”), finally gets to slap on some red lipstick, big hair and leopard prints with attitude to match. Though he doesn’t get much screen time, gruff ol’ grandpa Stacy Keach owns the movie’s most powerful hanky-grabbing moment. Keeping vigil at Mia’s bedside, he begs her to hold on while also giving her the OK to let go.

So which does she choose? If you don’t want to know yet, don’t read any of the spoiler-filled disclosures by people who have read the book, now running rampant online.

It’s safe to reveal that the movie ends on an abrupt, somewhat ambiguous note, sparking a “Huh? What?” ripple of murmurs from a preview audience. If that keep-’em-guessing strategy had been tapped more often throughout, it might have elevated this effort from merely acceptable to exceptional.