A fast-growing protest by high-school students over long hours, teacher shortages and decaying facilities has escalated into “urban violence”, the French government has warned, as it said many schools would switch to online classes to protect teachers and pupils.

The prime minister, Sébastien Lecornu, called a crisis cabinet meeting on Thursday and told ministers to cancel travel plans to deal with the demonstrations, which erupted in the Paris region last week but have snowballed across the country.

Lecornu’s office said: “This upsurge in violence makes it virtually impossible to maintain the continuity of public education and is exposing staff and students to danger.” The education minister, Édouard Geffray, is to meet pupil representatives on Friday.

About 900 demonstrations, blockades and related incidents were under way outside high schools around the country, police said, with more than 160 shut down as a result. Access was also blocked to about 30 university campuses on Thursday, including in Paris and Marseille.

Nearly 3,000 lycée students – aged between 15 and 18 – have been detained since the start of the protests, including 1,949 on Thursday alone, the interior ministry said. More than 300 police and dozens of firefighters, teachers and pupils have been injured.

The pupils’ grievances range from overloaded timetables and overcrowded classes to insufficient teachers and unbearable temperatures during heatwaves earlier this year. Riot police have been deployed to confront the increasingly violent protests.

Injuries have mainly been from hurled objects but also from police-fired riot-control projectiles, with the interior ministry announcing an internal police investigation after accusations of excessive force and brutality by officers against three pupils.

Rubbish bins have been set on fire outside many schools and more serious arson attempts have been reported inside some buildings. Fire damage in one school in Nantes in the north-west was so extensive that it will stay closed for a month, authorities said.

In the northern city of Caen, 16-year-old Maelle said she wished those setting bins on fire would protest “with words”. Students were protesting “against this government that doesn’t invest in heating, air conditioning, or teachers’ salaries”, she told AFP.

In Strasbourg in the north-east, tram services were temporarily suspended to “restrict the movements of rioters and demonstrators”, the regional prefect said, after protesting students obstructed the tracks with crowd control barriers.

“There are 35 of us per classroom,” one 17-year-old Strasbourg student said. “There aren’t enough toilets, and there are water leaks in some rooms. The building is falling apart, the cafeteria is too small. There’s zero budget, zero resources.”

Several bin fires were also reported in the southern city of Toulouse; while in Lille, young people wearing black hoodies destroyed a bus stop. In Marseille, there were attempts to light fires inside two schools. In one, a teacher was splashed with petrol.

While many teachers and several headteachers have reported being physically threatened and sometimes attacked, some support their pupils. Juliette Estiville, a high-school Spanish teacher in Paris, said that the police response had disgusted her.

“I’m actually here just to provide protection, to form a barrier between the police and the students,” she said. “Today went well, it’s going well, but I think that the teachers who come to the blockades are really there to protect our young people.”

The government has accused the radical left La France Insoumise (LFI) party of encouraging the protests, with several LFI MPs and mayors, including Bally Bagayoko, the LFI mayor of the Paris suburb of Saint-Denis, cheered at demonstrations.

The party’s candidate for the 2027 presidential election, Jean-Luc Mélenchon, insisted on Thursday that LFI backed “non-violent methods of struggle” and added that his “greatest worry is an unbearable level of police violence against these young people”.