A 15-year-old boy killed a fellow pupil and wounded three others in a stabbing attack at a high school in the western French city of Nantes on Thursday before he was overpowered by teachers, police said.

BFM TV reported that the student who died was a girl and that the three injured students were boys. Police did not confirm the age or gender of the victims.

Teaching staff subdued the attacker at the Notre-Dame-de-Toutes-Aides, a private Catholic school, before law enforcement officials arrived, a police spokesperson said.

She said there was nothing to indicate a terrorist motive.

Students were held inside the school after the midday attack but allowed to leave in the mid-afternoon under police protection. Dozens of parents waited outside.

"We're waiting to be able to hold them in our arms ... to help them deal with the stress this will have caused," said Nicolas, a parent at the school.

A classmate of the boy told reporters that the attacker had expressed Nazi sympathies.

"He spoke of Nazi ideology. We thought he just said that to make people laugh. ... What we heard is that he wanted to bring back the Nazi ideas of Hitler," she told reporters outside the school.

The classmate also said that shortly before the attack, the attacker had sent a long email to the entire school.

French media published excerpts of the mail, which they said had an environmental and anti-globalisation message but did not mention a possible attack.

Interior Minister Bruno Retailleau, a conservative, gave no details about the attacker's background or motives.

"The general climate of laxism and a lack of order and hierarchy is what leads to this kind of violence," he told reporters in Nantes.

Nantes Mayor Johanna Rolland, a socialist, said it was too soon to draw political conclusions.

"The mental health of the youth of this country is an issue that needs to be raised," she said.

The Nantes prosecutor will hold a press conference about the attack on Friday.

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