It’s not easy being a low-budget horror comedy. Horror fans will put up with cheap thrills; they’re much tougher on cheap giggles - Shaun of the Dead being the exception to the rule. And Jon Wright's teen slasher movie - set in a British grammar school - isn’t as clever or funny as Shaun. Still, it’s diverting, occasionally moving, and has two secret weapons, teen heart-throb Alex Pettyfer and newcomer Tuppence Middleton.

Middleton plays Oxford-bound head-girl, Justine, who has spent most of her school years working, sucking up to teachers and hanging around with Helena (the brilliantly spiky Mary Nighy). The latter is the kind of girl who organizes “Keira Knightley film evenings”. (Keira an icon for sapphic swots? Yes, it makes perfect sense!)
Justine now feels ready for an image change - she’s being courted by the “it” group, led by Bradley (Alex Pettyfer) - but suddenly finds herself at the centre of a string of deaths, which start with the suicide of an overweight, asthmatic “loser” called Darren Mullett (Calvin Dean). The latter, we discover, was being bullied by Bradley’s gang. Has the prey turned predator? And if so, whose side is our heroine on?
Justine is a complex character - sensitive yet smug, self-serving yet idealistic - and Middleton gets the balance just right. Surrounded by talented youngsters (including Skins’ April Pearson) she dominates the landscape effortlessly.

Pettyfer, meanwhile, is a revelation. He oozes spite - his very features, here, have a weaselish symmetry - which will horrify the tweenies who thrilled to his bland blondeness in 2008s Stormbreaker.

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Bradley - rich, horny and obnoxious - is definitely a stereotype, but the script injects something new into the formula, which Pettyfer sensibly exploits. At one point, Bradley calls his dumb best-friend a moron and, when the friend objects, our villain guffaws, genuinely amused. For an electrifying second, we get a glimpse of the real, bored-out-of-his-mind Bradley. He could have been a nice nerd. Alas, wealth, looks and laziness intervened.

The film-makers seem determined to play with the idea of perfection (Bradley’s room is full of pictures of naked girls; the camera lingers on (italics) Pettyfer’s (italics) much-hyped flesh).

It’s as if Wright is saying: pin-ups are part of the problem; put one person on a pedestal and another is bound to get crushed. Maybe Pettyfer agrees. Or maybe his career is in trouble and he’s trying to rebrand himself. Whatever, he’s a joy to watch.