The burnt body of Agnès Marin was found on Friday evening in woodland close to the school in the southern town of Chambon-sur-Lignon.
Her remains were discovered following a two-day search after she went missing on Wednesday. She had been sexually assaulted before being murdered.
In a statement to reporters the local prosecutor said the attack on Agnès Marin was “particularly violent and brutal.”
“This child was killed in atrocious conditions,” he said. The 13-year-old girl was sexually abused while being threatened with a knife before being killed and her body burnt.
In a shocking development, it was revealed that the 17-year-old boy who confessed to her murder had already been convicted of a similar attack and rape in 2010, in which the victim survived. He was imprisoned for four months before being released at the end of last year.
Agnès’ parents, Frédéric and Paola Marin, spoke of their “horror” at the murder of their daughter and accused the school of making a “huge error” by letting in the boy, known only as Martin.
The school defended itself on Sunday, claiming that while it knew the boy had been in prison, it was not made aware of the exact crime he had committed.
Claiming to be “astonished” at a “feckless and irresponsible” system that did not give them the full facts, the school’s director told a press conference they had “found out at the same time as Agnès’ parents this awful truth.”
Both pupils attended the College Cévenol, one of France’s best-known private schools and a popular choice with international students.
An American website for “Friends” of the school says that “for more than 60 years the College Cévenol has fostered international understanding and peace by bringing together students from many lands and cultures. The school’s original purposes – peace and cross-cultural understanding – inspire the programs today.”
The school was founded in 1938 on protestant principles and is famous for sheltering and saving Jewish refugees during the Second World War. It charges fees of up to €12,000 ($16,400) a year.