It’s very rare to find the fractious Cannes audience in total agreement about anything by this time in the festival, least of all a film. But Laurent Cantet’s exceptional high school docu-drama " The Class " has done the seemingly impossible, unifying an irascible group of people who could argue about anything from the best pizza in town to the correct pronunciation of Nuri Bilge Ceylan’s name. The consensus opinion: The Class is a masterpiece.

Who could imagine that a film about the exchange of ideas; a quicksilver battle of wits which debates everything, from the merits of African football to Plato to the correct definition of the word ‘skank’, could be so exhilarating? The film is humbling, affecting and utterly gripping.

Cantet’s movie came from a seed of an idea: that it would be interesting to set a film in a French urban junior high school. His early research brought him into contact with the writer and former teacher Francois Bégaudeau, who was then promoting his latest book, "Entre Les Murs " , a scaldingly honest and even-handed account of his own experiences as a French teacher.

Cantet, who gained much respect but less box - office clout for his naturalistic microcosms of French life in crisis, "Human Resources " and "Time Out " , could be described as a Gallic Michael Winterbottom. The pair share left - wing political sensibilities and a light, documentary - accented approach to film - making. Both have also used non-professional actors to impressive effect. But with "The Class " , Cantet has produced a film which, like Nicolas Philibert’s "Etre et Avoir " , the documentary about a rural French primary school, has the potential to cross over to an audience far beyond the limited arthouse sphere.

The film’s strength is that, without being sanctimonious, sentimental or didactic, it makes piercing observations about contemporary multicultural France – and as such it has a resonance that extends far beyond the school gates in Paris’ 20th arrondissement. Much of the film’s vibrant authenticity comes from the fact that the kids are real students from Francoise Dolto Junior High – Cantet work-shopped extensively with them over the course of a school year to develop the characters and the story – and that the teachers and parents are, in most cases, their actual teachers and parents. The role of the main teacher is filled by Bégaudeau, playing himself. With admirable restraint, he is not painted in the inspirational "Dead Poet’s Society " mould, but rather as a talented, committed but flawed character whose occasional misjudgements can have far-reaching ramifications in the lives of his students.

Infused with the energy of its subjects and captured by the light-footed cinematography of three cameramen working simultaneously, it’s full of significant moments. But for me, three stood out as exceptional. In one, a teacher loses his rag spectacularly in the staff room while his colleagues quietly look on, recognising in themselves his impotent frustration. Later, when Bégaudeau asks the students what they have learnt that year, comes the revelation that troublesome Esmeralda has secretly been reading Plato. Shortly after is the devastating moment when one girl claims to have learnt nothing - stricken and wide-eyed, her face articulates her fear about the future more expressively than anything she could say.

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