A 14-year-old girl has died of a heart attack in eastern France after her school locked down to protect itself from a knife attacker who lightly wounded two other girls, an official said on Friday.

The teenager "was rescued by teachers who were very fast to call the fire department. She died at the end of the afternoon," education official Olivier Faron said.

The girl's middle school in the village of Souffelweyersheim closed its doors on Thursday afternoon after a man stabbed two other girls aged 7 and 11 outside a nearby primary facility.

"Sadly this pupil underwent an episode of very high stress that led to a heart attack," Faron said.

A mother outside the middle school on Friday morning said her son in the first year of secondary had also been scared during the lockdown the previous day.

"Whereas in the primary school they made it more like a game, perhaps here it was a little too direct," Deborah Wendling said.

"He thought there was an armed person in the school. They could hear doors slamming, but it was just other classrooms locking down."

Faron defended the teachers.

"There is no perfect solution," he said.

But "we will analyze in depth what happened. If there are lessons to be taken from this, we will take them."

The two girls hurt in the attack were discharged from hospital on Thursday evening with only light wounds.

Police have arrested the 30-year-old assailant, and a probe has been opened into "attempted murder of minors", the prosecutor's office said.

It was not immediately clear what had motivated him, but it did not appear to be "a terrorist act", it said.

He was "psychiatrically fragile" and appeared to have stopped his medication.

The incident follows a series of attacks on schoolchildren by their peers, particularly the fatal beating earlier this month of Shemseddine, 15, outside Paris.

French Prime Minister Gabriel Attal on Thursday announced measures to crack down on teenage violence in and around schools.