France has suspended the use of stun grenades for policing student protests that have swept the country, after a 15-year-old’s hand was blown off earlier this week.

Interior Minister Laurent Nuñez said he had requested the pause until the facts of Monday’s incident in Lens had been “clarified”, with conflicting accounts offered by protesters and officials.

Several groups have complained of police using disproportionate force towards teenagers.

The disruption continued early on Wednesday with more than 500 schools fully or partially shut, and Prime Minister Sébastien Lecornu is due to speak about the school protests in an evening address.

Education Minister Édouard Geffray, appearing before France’s National Assembly, said 24 schools had been “set alight or vandalised” to such an extent that they would be shut for several weeks, affecting 30,000 pupils.

Hundreds of thousands of people took to the streets on Tuesday to call for better school conditions and in some cities tear gas and water cannon were deployed. The interior ministry said 488 people were arrested and 43 members of the security forces were hurt.

Nuñez told French radio RTL that police watchdog IGPN had opened 18 investigations to determine whether officers had been “proportionate” during the demonstrations, which have spread from high school pupils to their parents, university students and unions.

“There is a police action that raises questions for all of us within the Ministry of the Interior; it’s what happened in Lens,” he said, and confirmed the use of stun grenades was being suspended “until the facts surrounding this case have been clarified”.

The 15-year-old boy’s hand was torn off after a so-called anti-encirclement grenade was fired in unclear circumstances near the Auguste Béhal high school in the northern city near Lille.

Authorities have said he picked up the grenade which then exploded, but students have alleged he was hit directly after an officer fired.

According to the local prefect, the incident took place when police were confronted by a group of 200-250 students throwing stones and fireworks.

The stun grenade is considered a non-lethal weapon, but sends out small rubber pellets when it detonates.

“When there is suspicion of disproportionate force, there are always judicial investigations,” Nuñez said, affirming that the government was “here to protect high school students”.

The national secretary of policing union UNSA said he was sceptical about the decision to halt the use of stun grenades, saying it had been based “on emotion”.

“We are dealing with increasingly violent people,” Jean-Paul Nascimento told FranceInfo, cautioning that without the weapon to disperse crowds there may be more clashes at close quarters. “That’s where there will perhaps be even more injuries,” he warned.

Concerns have also been stoked by the case of a 14-year-old hit in the face with a tear gas grenade last week. The boy’s lawyer said he had lost several teeth and suffered a fractured jaw in the incident at Saint-Ouen-l’Aumône, north-west of Paris, and a police officer has been charged.

In a separate development, a 16-year-old suffered a serious hand injury outside a school in Strasbourg on Wednesday, reportedly while handling a mortar firework.

Prosecutors said on Wednesday that an investigation for attempted murder had been opened after a 17-year-old student protester complained an officer “held his head underwater” while trying to arrest him near a river in the north-eastern city of Belfort. French reports say the boy has himself been charged with aggravated assault for allegedly attacking the officer.

The prime minister has condemned violence seen at some of the protests which began in mid-September – noting on Tuesday that 215 students, 85 staff, and 715 police officers had since been hurt.

More than 6,500 people have been stopped or arrested in that time, according to the interior ministry.

President Emmanuel Macron held an emergency meeting late on Tuesday with Lecornu and other key ministers to find a way out of the crisis after almost a quarter of all lycées were disrupted by the day’s protests – with government figures putting the number of protesters at more than 250,000.

Students’ unions put the number at 450,000 and have said a fourth round of protests is planned for Thursday in Paris and other cities.

The head of the USL high school union, Ryad Rani, told French radio on Wednesday: “We are telling M. Nuñez, calm your police officers down and call for restraint.”