France is reeling after a 17-year-old boy raped and murdered a girl at a chic boarding school while conditionally freed on charges of rape a year ago.

"It's absurd. It could have been avoided with a little less negligence," the dead 13-year-old's mother, Paola Marin, told Europe 1 radio as a nation struggled to comprehend how the gruesome crime could have happened.

The girl, Agnes, was a pupil at the College-lycee Cevenol International in central France.

Her burned body was found after she was raped and murdered in what the prosecutor said was "an extremely violent and brutal way".

The girl's family, the school and French educational and judicial authorities are all asking the same question: how could such a crime have taken place given the boy's history?

The school prides itself on teaching non-violence and the town where it is situated, Le-Chambon-sur-Lignon, is famous for having protected French Jews from the Nazis during World War II.

Did the school know that the boy, identified as Mathieu, had been charged with rape in the south of France in 2010? Why did the courts allow him to go to a "normal" school despite his past? Did the system fail?

Prime Minister Francois Fillon said "we must clarify possible malfunctioning of the penal system" and make prevention of repeat offences "an absolute priority".

The killing reignites debate in France over how to halt repeat offences, after right-wing President Nicolas Sarkozy brought in a law to severely punish such crimes after a spate of headline-grabbing cases.

Agnes vanished on Wednesday after Mathieu allegedly lured her into a nearby forest, where she was killed on the same day. Her body was found on Friday after the boy confessed to "raping and burning" her.

The girl's parents have accused the school of failings, with her father, Frederic Marin, insisting the school knew of the boy's past.

Anges's mother said: "The school should have been a bit more vigilant."

But the school, whose motto is "humanism and tolerance", has blamed the judicial system. Its management said it did indeed know that Mathieu had spent four months in prison but did not know exactly why.

"If I had known, I would not have accepted him in our establishment, because we're not equipped," headmaster Philippe Bauwens said.

The boy, described as a brilliant pupil in the final year of high school, was in August 2010 charged with raping a minor and spent four months in jail pending trial before being freed under judicial supervision.

Prosecutor Jean-Yves Coquillat said the justice system did what it had to do, with the boy only being released under strict conditions: he was treated by a psychiatrist in a nearby town and by a psychologist at the school.

The prosecutor's office pointed to psychiatric evaluations that said the boy could be "rehabilitated and did not show signs of being dangerous".

Many blamed a lack of communication between different state services.

"There was a serious breakdown between what the school's management knew and what the national education system and the judiciary knew," said Anne-Sylvie Debard, a member of the school's board of governors.