France will mobilize up to 7,000 soldiers to increase security around the country after a teacher was fatally stabbed and three other people wounded in a school attack by a former student suspected of Islamic radicalisation, the president's office said on Saturday.

Some children and personnel returned to the Gambetta-Carnot school in the northern city of Arras as it reopened Saturday morning, after a schoolday attack Friday that rattled France in a context of global tensions over the Israel-Hamas war.

Counterterrorism authorities are investigating the stabbing, and the suspected assailant and several others are in custody, prosecutors said. The suspect is a Chechen who had attended the school and had been under recent surveillance by intelligence services for radicalisation.

The government heightened the national threat alert, and President Emmanuel Macron ordered up to 7,000 soldiers deployed by Monday night and until further notice to bolster security and vigilance around France, his office said.

The “Attack Emergency” threat posture allows the government to temporarily mobilise the military to protect public places among other measures.

At the school Saturday morning, police stood guard as adults and children trickled in. Classes were cancelled, but the school reopened for those who wanted to come together or seek support. One mother said she came with her 17-year-old daughter in a show of defiance against extremism, and to overcome the fear of returning to a site where children were locked down for hours after the stabbing.

The attacker's exact motive remains unclear, and he is reportedly refusing to speak to investigators.

For many in France, the attack echoed the killing of another teacher, Samuel Paty, almost exactly three years ago near his Paris area school. He was beheaded by a radicalised Chechen later killed by police.

The suspect in this week's attack had been under surveillance since the summer on suspicion of Islamic radicalisation, French intelligence services told the Associated Press. He was detained Thursday for questioning based on the monitoring of his phone calls in recent days, but investigators found no sign that he was preparing an attack, Interior Minister Gerald Darmanin said.

French intelligence suggested a link between the war in the Middle East and the suspect's decision to attack, the minister said. He said authorities have detained 12 people near schools or places of worship since the Hamas attack on Israel, some of whom were armed and were preparing to act. France has heightened security at hundreds of Jewish sites around the country this week.

The prosecutor said the alleged assailant was a former student there and repeatedly shouted “Allahu akbar,” or “God is great” in Arabic during the attack. Prosecutors are considering charges of terrorism-related murder and attempted murder against the suspect.

The dead educator was Dominique Bernard, a French language teacher at the Gambetta-Carnot school, which enrols students ages 11-18. Another teacher and a security guard were in critical condition with wounds from the stabbing, police said. The counterterrorism prosecutor said a cleaning worker was also injured.

Announcing that the school would reopen Saturday, Macron urged the people of France to “stay united”.

“The choice has been made not to give in to terror,” he said. “We must not let anything divide us, and we must remember that schools and the transmission of knowledge are at the heart of this fight against ignorance.”