Creepy kids are one of horror's richest tropes, having birthed such fantastically frightening films as "The Bad Seed," "The Exorcist" and "Let the Right One In," as well as campier yet still freaky fare like "Pet Sematary," "Children of the Corn" and "Mama." So I had hope for "Cooties," a horror-comedy that transforms booger-eating metaphorical ankle-biters into bloodthirsty literal ones. Unfortunately, this zombie-kid flick lacks bite.

The directorial debut of Jonathan Milott and Cary Murnion starts off strong, with a title sequence so disgusting that I was dry heaving by its final shot. In a nightmarish parody of the opening of "Willy Wonka and the Chocolate Factory," a montage offers a look not at the creation of mouth-watering sweets, but rather the nauseating process that transforms a sickly bird first into meat mush and then into chicken nuggets. Climaxing with an unsuspecting pig-tailed blonde biting into a deep-fried morsel laced with a thick, viscous vein of disease, the sequence not only sets up the film's plague, but also might be enough to put you off the cafeteria staple for good. With my eyes watering and my stomach flipping, I was primed to expect goods that "Cooties" just doesn't deliver.

Continuing a horror-film streak that includes "Maniac," "Grand Piano" and "Open Windows," Elijah Wood stars as Clint, pretentious aspiring novelist who couldn't hack it in New York City, and so returns home to Fort Chicken to take a job as a substitute teacher at his old school. His first day goes from embarrassing to apocalyptic when the aforementioned nugget-muncher bites her bullying classmate on the face, triggering a pandemic that turns her prepubescent peers into merciless, flesh-eating monsters.

Trapped inside the school, Clint strives to survive alongside a staff made up his ever-chipper childhood crush (Alison Pill), her macho gym-teacher boyfriend (Rainn Wilson), a high-strung creationist (Nasim Pedrad), a closeted man who has no traits beyond cargo-shorts and lame gay jokes (Jack McBrayer), a proverb-spouting Japanese janitor (Peter Kwong), who naturally knows martial arts, and a socially awkward science teacher (Leigh Whannell).

It's an ensemble so big you expect it will be whittled down by schoolyard carnage. Yet the script from Whannell ("Insidious") and Ian Brennan ("Scream Queens") rejects this zombie-movie trope. Instead, mostly random extras are offered up to the teeth of its ravenous tykes, inciting McBrayer to bellow "Where did she come from?" when a rabid student misses Pedrad only to accidentally land on a previously unrevealed faculty member.

This mystifying aversion to killing off its main cast means "Cooties" is bloated by too many characters and their uninspired shtick. Struggling with thinly sketched roles, McBrayer and Pedrad have little to do but toss out hit-or-miss one-liners and make faces at the massacre around them. Kwong is given a racist stereotype and eye roll-inducing "jokes." Whannell, grating as the comic relief in the "Insidious" franchise, is surprisingly funny here as a flummoxed geek trying to make sense of this outlandish outbreak. But both he and Wilson suffer from catchphrase callbacks that get no funnier with their dogged repetition. (Yeah, "dual rear wheel well" is hard to say. Move on.)

A regrettable chunk of teacher time is spent wallowing in a tedious love triangle between Wood, Pill and Wilson. Wilson relishes in playing a past-his-prime jock with a lot of pent-up rage, but fumbles the hurried heartfelt monologues the script shoves his way. Wood's anti-hero is only made more unlikable as he goes from whining about their circumstances to waxing on about his novel to flirting with his traumatized but chronically nodding love interest. And Pill is wasted as a smiling moron who’s sole purpose is to give Wood and Wilson an object to fight over. (At least in "Snowpiercer" her smiling moron teacher got to be scary and ass-kicking.)

The real stars are the child zombies. Montages of moppet mayhem are delightfully demented, revealing intestines used as jump rope, eyeballs as marbles, and a decapitated head for a game of tetherball. The hordes of young actors are wonderfully horrific, delivering guttural growls, ghoulish grins and grim giggles. The ironically named Sunny May Allison is perfection as the point of infection, Shelly. Covered in boils, boogers and gore, she snaps from a dull stare to a menacing glare when prey is in reach, making for macabre fun.

But when dealing with adults killing kids, there's a balance. And once it's clear these kids are "infected persons" and not "reanimated corpses," the dark delights of watching the teachers beat, batter and flat-out murder their students turns a chilling corner. It becomes gross, disorienting and increasingly sad.

The biggest problem of "Cooties" is that, much like its Ritalin-popping tweens, it's unfocused. The tone switches from loony humor to earnest tenderness to repellent gross-outs at breakneck speed. Overstuffed with characters that lack an arc or motivation beyond "survive," its plot meanders, crippling any possible momentum. The teacher's clunky flee/hide/bicker cycle oozes with jokes, but less than half of those are actually funny. The gore is good and deeply gross, tut that's not enough to put up with a movie that feels long, even at 96 minutes.

"Cooties" opens today in select theaters and on VOD.