The Cannes film festival has awarded its top prize to a French classroom drama with a cast of amateurs in a banner year for movies with a conscience.

In a popular choice, the jury led by US actor-director Sean Penn gave the festival's coveted Palme d'Or to "The Class", featuring a multicultural cast of first-time teenage actors all plucked from the same Parisian secondary school.

The picture, shot in documentary style and based on the bestselling memoir of French teacher...