When Laurent Cantet heard his name called during the awards ceremony at last month's Cannes film festival, he was not entirely surprised. The Class (Entre Les Murs), Cantet's Palme d'Or winning film, had been tipped to take a prize when the busload of schoolchildren who appear in the film was called back to Cannes just one hour after it had left town on Sunday morning. Still, Cantet was not expecting to walk away with the first French Palme d'Or since Maurice Pialat's 1987 win for Sous Le Soleil De Satan.

"I thought it would be a special prize for the kids," Cantet recalls. "But, as more and more prizes were being handed out, I started to dream it would be us."

The film-maker is well-known for such award-winning works as Human Resources and Time Out, both contemporary dramas about the politics of work and family. For The Class, Cantet turned his lens on a subject even closer to his heart: a school. Cantet's father was a teacher and his own children are the same age as those in the film.

The film began to take shape a few years ago when Cantet appeared on a radio show where another guest, Francois Begaudeau, was talking up his autobiographical book, Entre Les Murs. It was about Begaudeau's life as a teacher in a tough Paris neighbourhood. "Hearing him read an excerpt, I felt there was a convergence between his book and my project," Cantet explains.

Cantet proposed to make a film "based around the book's story", albeit not a straight adaptation. Cantet likes the term "a documented film". The events in the film are based on real incidents but the students are acting roles. Cantet and Begaudeau co-wrote the script with Robin Campillo.

With Begaudeau, who also appears in the film as himself, Cantet organised weekly workshops in a Paris high school where students were invited to come and improvise. From November 2006 to April 2007, about 50 young people participated.

Of those, the students who ended up in the film are "the 24 who were the most faithful and interested in the project", says Cantet, who believes that some of them, particularly Wei Huang, Franck Keita, Esmerelda Ouertani, Rachel Regulier and Carl Nanor, may go on to have successful acting careers. For all of them, The Class was their first professional acting job.

Now that Cannes is over, the director will be mobilised to support the film in its international roll-out. Paris-based Memento Films International has sold the film extensively, including to Sony Pictures Classics (US), Curzon Artificial Eye (UK), Golem (Spain) and Jin Jin (South Korea). Haut Et Court is opening it in France in September.

"I'm preparing an autumn filled with travel," says the director. "I am surprised to realise in the US how many people know my films, so to see the American reactions interests me a lot... I'm especially reassured because we were told the film was too French to touch an international audience and now I feel like that fear wasn't necessary."