The girl, named as Louise, was found stabbed to death in the Essonne region south of Paris in the night of Friday to Saturday, police said.
She had been missing since leaving school on Friday afternoon and was found just a few hundred metres from her school.
According to a police source, who asked not to be named, she had been "hit more than 10 times by an object that could have been a knife", particularly in the torso and neck.
French police arrested a man aged 23 on suspicion of murdering a child aged under 15, prosecutors said.
His mother, aged 55, has also been detained in the case on suspicion of failing to report a crime.
A couple in their twenties were detained Saturday as part of the investigation but were released without any further proceedings at this stage, prosecutors said.
While the circumstances of the killing remain unexplained, it comes at a time when law and order, and in particular crime against children, are major issues in French politics and society.
"An innocent life was torn away from life and the love of her family. We are in the process of the investigation, but sadness and anger are overwhelming every French person," said Interior Minister Bruno Retailleau.
Some 120 members of the security forces were deployed on Sunday at the Longjumeau wood where the body was found, seeking any clue that could lead police to the perpetrator, prosecutors said.
According to a police source, the victim's mobile phone was found close to the body.
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