"There was a field of such powerful, emotional, moving movies and performances," he said.
"There were so many times that we thought it just can't get better."
The Class - based on an autobiographical novel by young teacher Francois Begaudeau - was praised by Penn as "an amazing, amazing film".
Director Cantet - who was joined on stage by his teenage cast - said he aimed to make a film that was "a reflection of French society - multiple, many-faceted, complex".
"Sometimes also with friction that the film does not try to cover up," he added.
Italian mafia movie Gomorrah, set in Naples and based on a book by Roberto Saviano, took the Grand Prix runner-up prize.
The third place jury prize went to Il Divo, Paolo Sorrentino's portrait of the country's former prime minister, Giulio Andreotti.
After the awards ceremony, the festival closed with the premiere of Barry Levinson's What Just Happened? - a tale of a fading Hollywood producer trying to lift his career, starring Robert De Niro, Bruce Willis and Sean Penn.
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