ROSNY-SOUS-BOIS, France — Laïcité, the concept of state secularism, is a defining principle of the French republic, right up there with the national motto of liberty, equality, and fraternity. Developed in the French Revolution, which targeted the Roman Catholic Church as much as the monarchy, laïcité governs the public life of a nation that sharply delineates the realms of Caesar and God.

But laïcité (pronounced lie-EE-see-tay) is under severe challenge from Islam, the vibrant, growing religion that arrived in France with post-colonialism. Islam does not easily accept the ban on the public exercise of religion, whether it is the full veil for women or gender-mixed swimming pools, Friday Prayer that overcrowds mosques or halal food in schools.

Now, in the aftermath of the Charlie Hebdo killings and an attack on a Jewish supermarket by Islamist gunmen, there are new questions about laïcité and whether it is being fairly applied in a France with an estimated 5 million Muslims — close to 8 percent of the population and making up the largest number of regular worshippers.

After the killings, there is a new government edict to reinforce teaching of laïcité in public schools, where religious education is banned. The edict is aimed especially at schools of heavily Muslim suburbs, where many children shocked the country by refusing to obey a national minute of silence for the dead of Charlie Hebdo, which they believe insulted the Prophet Muhammad.

France has even declared Dec. 9 a new Day of Laïcité; teaching candidates will be tested on their understanding of the concept of secularism. Beginning in September, students and parents must sign a charter of laïcité to “demonstrate their willingness to respect it.”

At a news conference in Paris this week, President François Hollande said that France would defend its central values, including secularism. The concept of a secular state is “nonnegotiable,” he said, calling laïcité “a guarantee for France” against threats both internal and external.

Jean Baubérot, a sociologist, called the new measures aggressive.

“This new laïcité, which I would call repressive rather than strict, is no longer that of 1905,” when France legally separated church and state, he said. “It risks being draconian and counterproductive, leading to feelings of victimhood. We don’t need such things in the current situation.”

Pierre-Eric Nahon, a head teacher in Noisy-le-Grand, a nearby suburb, says he has an increasingly difficult job. Teaching laïcité was easy when his students were largely homogeneous and few were religious.

He estimates that more than a third of his students are Muslim, and he says he is not aware of a single Jewish student in his school or any other public school in the larger governmental area of Seine-St.-Denis. After the government’s new orders to promote laïcité, he and his staff are debating how to teach it. Should it be redefined, he asked, to allow religious education, including about Islam?

“We have to absolutely talk about this,” said Nahon, 51. “We cannot continue as we did.”

In theory, laïcité promotes egalitarianism, keeping religious belief private. But critics say that, in practice, it has become a pretext for exclusion and discrimination.

Benoist Apparu, a legislator and former secretary of state for housing, said that the law was outdated.

“We’ve got to stop with this secular totalitarianism,” he said.

Reforming the law “should not be taboo,” he said. “We need to accept the debate.”

Youths living in the suburbs “see laïcité as a way to push them away,” said Louis-Georges Tin, president of the French Black Coalition and République & Diversité, a research body.

“In recent years, it has become a way to discriminate between Muslims and Christians,” he said.

Muslim students may not wear headscarves, he said, but some schools hold Mass every day, and nearly every French state holiday is a Roman Catholic holy day. Halal food is not permitted in school canteens, although non-pork meals are often available, but schools can offer fish dishes on Friday.

“Either you allow everything, or you ban everything,” said Tin.

Nahon acknowledged the discrepancies. While he supports a strict application of laïcité, the school adheres to the Catholic liturgical calendar and holds Christmas parties every year, and students eat crepes for Mardi Gras and chocolates for Easter.

Laïcité was formalized in the 1905 law, which since has meant that churches and synagogues built previously are state property and maintained by public funds. But Islam came later, mosques get no state funding, and the state has struggled to apply laïcité to public schools, beaches and sports halls. (Alsace-Lorraine, German in 1905, operates under the Napoleonic Concordat that allows religious education but does not include Islam among the religions that are studied.)

Regulations aimed at Islam have increased. A law banning headscarves (and other religious symbols worn conspicuously) in public high schools was passed in 2004. Another law banning the full-faced veil in public spaces was passed in October 2010.

Women are also banned from wearing headscarves while accompanying students on school trips if they “perturb scholastic activity.” Minarets, typically part of a mosque, are rarely allowed. Overcrowded mosques have forced Muslims to pray in the streets, which Marine Le Pen of the conservative National Front has likened to the Nazi occupation — and an occurrence mayors have the authority to ban.

M’hammed Henniche, the head of the Union of Muslim Associations of Seine-Saint-Denis, calls this application of laïcité aggressive and unfair. Muslims can live with a “neutral laïcité,” he said, but the rules keep changing. When the government tried to restrict halal abattoirs, or slaughterhouses that adhere to Islamic law, and require sermons only in French, he said, the measures were abandoned not because of Muslim opposition, but because of opposition from Jews in the first case and Russian and Greek Orthodox believers in the second. Jews also blocked a government move to force people to choose a single nationality, he said.

There are not nearly enough mosques, either, and the government does not own or subsidize them.

“Mayors consistently block the construction of new mosques,” Henniche said.

But a few, he acknowledged, as in Bordeaux and Nantes, had tried to help mosques by arranging long-term leases on public land. The authorities change the rules all the time, he said.

“Tomorrow they’re going to come and tell me, yeah, your name is M’hammed, that’s no good, you must chose a first name like Jean-François,” he said.

Learning about laïcité in schools is fine, he said.

In December, the Interior Ministry had a meeting about radicalism and asked for proposals.

“We said you should value Islam to children,” Henniche said. “So a child is told that Islam is one of the great religions like other religions. So the child will be proud of his origins and of his parents. But when one mocks the religion of his parents, he’s going to hate you,” he said.

The law of 1905, said Roger Cukierman, the director of the Representative Council of French Jewish Organizations, “was an agreement in a Christian country among mainly Christians,” long before the influx of Muslim workers and their families, but it recognized Judaism.

While laïcité meant keeping religion in the private domain, “it’s more and more difficult, because the Muslim minority is requesting the ability to pray every few hours and to have halal food, including in private enterprises,” Cukierman said.

Voltaire wrote that religion was on a diminishing road, but it has returned with a vengeance, said Dominique Moïsi, a French political scientist.

“Laïcité has become the first religion of the Republic, and it requires obedience and belief,” Moïsi said. “But I care more for democracy than for the republic,” he said. “To play Voltaire in the 21st century is irresponsible.”