Films can be innovative in different ways. Sometimes they invent a new way of staging and shooting traditional dramas (see Dogme), and sometimes it's the story that is original while the film's visual style remains relatively mainstream (see most films with which Charlie Kaufman has been involved).

But occasionally a film comes along that forces its viewers to think about narrative cinema itself, about what it does, what effects are achieved and how these are generated. A good example is Laurent Cantet's deserved Cannes Palme d'Or winner The Class.

Cantet's film is, ostensibly, about a boisterous, multi-ethnic junior high school class in a tough inner-city suburb of Paris - its personalities, problems and the challenge of teaching street-smart but academically unmotivated kids who attend school mainly because they are legally obliged.

'A microcosm of the world'

But the film is really about the uneasy relationship between democratic debate and commands from on high; between open societies and closed authoritarian ones. The class in The Class is a microcosm of the world outside, and the intelligent, funny, liberal teacher who presides over it is a man whose good intentions are hijacked by the coercive system in which he works and by the remnants of social snobbery he has not yet managed to flush out. The film's French title, Entre Les Murs ('between/inside the walls') is resonant in this respect: the walls are those of the classroom and of Paris itself, but also the walls of prejudice and the invisible barriers that divide teacher and pupil.

One working definition of a good film is just this: it's a film that has something going on beneath the obvious storyline. We don't need to be able to fully articulate it, just feel that it's there. The Class clearly qualifies as a good film according to this benchmark.

But there's a third step beyond 'what it's about' and 'what it's really about' that can push a film towards greatness. This is when it embeds its theme in the use of cinematic technique and discourse that a novel or a play cannot manage - and so makes the medium resonate, and makes its audience think about the medium.

Though it seems to contain elements of documentary, The Class doesn't fiddle with format like another of this year's outstanding Cannes Competition titles, animated documentary Waltz With Bashir; nor does it parody the conventions of non-fictional film-making in the style of a Christopher Guest mockumentary. It's not the sort of gritty, handheld docudrama we're used to, either: the HD cameras are mostly fixed, the camerawork and shallow focus are highly cinematic. And we never feel the director is trying to give the impression his non-actors are not acting. Actually, The Class derives much of its authenticity from the fact the film's cast of real students are clearly acting - but they're acting themselves or facets of themselves.

So here's one way The Class makes us think about cinema. Even without knowing the details of the workshops that generated these classroom debates and dramas, we sense just by watching the film that a way has been found to challenge the normally authoritarian, inflexible, hierarchical process of feature-film production - to let the cameras roll, let the actors improvise and re-enact the film's debate about democracy in the classroom through the freshness and openness we feel in its structure. It's permeable, where most films are closed systems.

Unorthodox storytelling

And here's another. Most films feed us stories in a fairly formulaic fashion. The inciting incident somewhere on page three of the script, the end-of-act-two setback: such dosing mechanisms ensure we get the medicine we're used to at the correct times. But The Class forces its audience to forage around for story in what at first seems a miasma of enjoyable but fairly plotless classroom talk. The fact the story kicks in with a vengeance around two-thirds of the way through forces us to go back and reassess those seemingly innocent exchanges, and discover that even here the seeds of crisis were lurking.

Without preaching, it teaches the audience to sit up and pay attention not to the big turning points, but the micro dramas hidden in every line and every glance.