Ever since the original “Night of the Living Dead” the zombie apocalypse has become a cinematic cliché, and in the last decades it’s exploded into a positive phenomenon—just witness the continued success of “The Walking Dead.” Spoofs are no longer rare, either—going back at least to “Shaun of the Dead” and “Zombieland” (the latter still one of the best). It’s gotten harder and harder to do something different with the premise, but this amiably cheesy Scottish picture manages the trick by imagining it in the vein of “High School Musical.” For a while, the idea works—until things go dark, with unfortunate results for both the characters and the audience.

The screenplay, expanded from a 2011 short film called “Zombie Musical” by the late Ryan McHenry (here given co-writing credit along with Alan McDonald), begins by introducing a group of friends at Little Haven High. Chief among them is the titular girl (Ella Hunt), who wants nothing more than to see the world, skipping college in the process—a prospect that irritates her widower dad Tony (Mark Benton), who’s also the janitor at the school, to no end.

Anna has two admirers among her classmates. One is John (Malcolm Cumming), the sweet-natured best friend who’s obviously in love with her. The other is Nick (Ben Wiggins), an arrogant bully with whom she broke up but who is still harassing her. And she has other pals: Lisa (Marli Siu), who is planning a sultry dance routine for the school’s Christmas show; her boyfriend Chris (Christopher Leveaux), a geeky aspiring filmmaker; and Steph (Sarah Swire), an American transfer student and social activist still pining away for the girlfriend she left behind.

All are introduced early on in bouncy numbers with instantly forgettable music and—if you can discern them—pretty lame lyrics, often backed up by stomping choruses of anonymous fellow students. It all has the feeling of a high-spirited off-off-Broadway show, with—of course—a villain scowling at everything. He’s the school’s headmaster Mr. Savage (Paul Kaye), a Captain-Bligh-on-land type with delusions of grandeur, an especially recognizable sort of fellow in the age of Trump.

Then the zombie epidemic strikes, with comic-tinged horrors occurring throughout the town even though at first Anna and Nick are blissfully unaware of them, being too absorbed in happy song and dance. Being attacked by a guy in a Snowman suit in the park finally gets through to them, and eventually they join their friends to make it back to the school, where Savage has set himself up not just as dictator but the arbiter of who will survive.

As the movie proceeds, it gets increasingly dark, and the innocuously freewheeling quality succumbs to something snarkier and less engaging. One can point to the place where “Anna” starts really going off the rails: “Soldier at War,” a song about the joys of killing zombies delivered by Nick and his gang of thuggish followers. Like the second-act opening that drove the stake into the heart of “Carrie the Musical” back in 1988—a little ditty titled “Out for Blood”—the number is not just bad but tasteless, and not in the funny, ghoulish way it’s aiming for. The movie never recovers.

Still, the exuberance and lollipop colors of the first half, as well as the likability of the young cast (Kaye, on the other hand, wears out his welcome pretty quickly), aren’t entirely cancelled out by the miscalculations of the last fifty minutes or so. Though “Anna and the Apocalypse” is a mixed bag, it offers some modest treats up front.