France is seeing its biggest youth protest movement in decades. Here is what is driving it.

The trigger: students began blocking high schools in mid-September. They complain of missing teachers, overcrowded classes, dilapidated buildings and collapsing ceilings. Some schools cannot cope with rising temperatures. Hundreds of schools have been blockaded.

The scale: Tuesday was the biggest day yet. The interior ministry counted 266,000 demonstrators. Unions claimed 450,000. Teachers, parents and unions joined the students. Marches took place in Paris, Marseille, Lyon and other cities.

The clashes: police fired tear gas and used water cannons in Paris. Nearly 500 people were detained. Rights groups called the force against teenagers disproportionate. The government says the response was proportionate and blames violent agitators.

The politics: the protests pressure outgoing President Emmanuel Macron. Far-left leader Jean-Luc Melenchon backs the students. More strikes are planned across the economy. New student protests are expected on Thursday.