The teacher was given emergency aid at the scene, but Bayonne prosecutor Jerome Bourrier said she died of her wounds.

The pupil was arrested and a murder investigation has been opened, he added.

A source close to the case said that, by the time the police arrived on the scene at around 9.50am, the attacker had been disarmed and other pupils isolated.

He had been carrying a blade some 10cm (almost 4 inches) long, they added.

The pupil seemed to have acted in a “moment of madness”, rather than on any “terrorist motive” or “resentment”, the source said.

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Ines, who witnessed the attack, said she did not really know the teenager. “We’re just in Spanish class together. But there had never been a problem between him and the teacher in class,” she said.

The school, Saint-Thomas d’Aquin, is a private and Catholic-based establishment close to the centre of Saint-Jean-de-Luz, which in summer is one of France’s best loved resorts on the sandy Basque country coast.

The teacher had long taught at the school and was “conscientious”, a representative from the FEP-CFDT teachers’ union said.

By lunchtime, pupils had started to leave the premises after being confined to their classrooms for around two hours after the incident. Anxious parents were waiting for them but only those parents of the class where the stabbing happened were allowed to enter the school.

France’s Education Minister Pap Ndiaye said he was “extremely upset” by the death of the teacher.

All schools would observe a minute of silence for her on Thursday afternoon at 3pm, he told the press outside the school.

Government spokesman Olivier Veran said he could “barely imagine the trauma that this represents at a local level and more generally on a national scale”.

The BFM television channel said that the attacker had locked the classroom door and stabbed the teacher in the chest.

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The channel quoted a source as saying that the boy then told another teacher that a “voice” had told him to carry out the action. The investigation was to seek to determine his psychological state and motives.

No details have been released concerning his background.

Such attacks at schools are generally rare in France but there have been growing concerns about the security of teachers. In the past 40 years, there have been fewer than a dozen deadly attacks in schools.

The attack in Saint-Jean-de-Luz is the first killing of a teacher in France since the October 2020 beheading of Samuel Paty outside Paris by an Islamist radical.

In July 2014, a 34-year-old teacher was stabbed to death in the southern town of Albi by the mother of a pupil. The perpetrator was later found to be legally irresponsible.

A Jewish school was targeted in the attacks carried out by Islamist gunman Mohamed Merah around Toulouse in 2012, with a teacher and three pupils shot dead.

In Saint-Jean-de-Luz, Maha Bargueche, a mathematics teacher from the Paris region who was holidaying in the area, placed a bouquet of flowers in front of the school “as a sign of support”.

“I’m very sad, it could have happened to me, it can happen to any teacher. That’s why I came immediately,” she said.