No existing cinematic mode can deal with the horror, complexity and broad-daylight actuality – contradictory yet symbiotic attributes – of a school killing spree. So The Dirties invents a mode that doesn’t exist. (Till now.) It’s part fiction, part improvisation, part mock-blog. Amateur movie scenes – director-star Matt Johnson filming at home and school with his best friend (Owen Williams) – are collaged together with stunt vérité sequences and a climax that seems unnervingly real while almost self-advertisingly faked. Actual schoolkids were used in the film; you may think misused. (Hoodwinking is seldom nice.) The film is messy, a little confusing, indisputably – at times – powerful.