That's F as in "Fail", which describes what troubled teacher Mr Anderson (David Schofield) is doing after a traumatic attack on him by a pupil.
He's also hit the bottle, lost his wife and alienated his daughter. Can he recover his nerve when the school is invaded after hours by a gang of faceless hoodies armed with knives and monkey-wrenches? Johannes Roberts shoots creepily down gloomy corridors and contrives moments of real suspense as the stalk-and-slash proceeds, though he slightly overdoes the balletic agility of the hooded assailants. But as a tribute to master-scaremonger John Carpenter this is a promising B+.