Except in the regions of Alsace and Moselle, which were part of Germany when France's secular school system was created in the 1880s, there is no religious instruction for the 10 million pupils in state schools.
But in the wake of attacks that have killed 241 people, claimed by radical Islamists, since January 2015, the state has begun to look again.
The national education minister has introduced the "secular teaching of religious facts".
Under the initiative, elements of religious studies have been included in geography, history, literature, arts and philosophy classes to "enhance understanding of our cultural heritage and the contemporary world."
Many teachers have taken it on board, though some have felt underprepared to teach religion in a society where religious practice is generally in decline, said the president of the Observatory of Secularism, Jean-Louis Bianco.
"Some teachers say 'I need to know the Koran, the Bible, the Torah'. But they are told that is not the case. They do not need to engage in confrontations with students who defend their religion."
Balancing act
The state has taken the initiative of offering training to teachers, including with the launch of an online course. But finding time is a struggle for busy teachers.
Pointing to a further barrier, Philippe Gaudin, deputy director of the European Institute of Religious Studies, said: "If you have teachers who speak all day long to students sitting in a chair, there is no room for innovation."
Teachers walk a fine line. In Indre, in central France, a primary school teacher was transferred after she was considered to have spent too long reading out Bible passages to pupils.
"Everything is a matter of balance," said Mr Bianco.
Knowing and believing
Children and teenagers are rarely hostile to religious teaching, according to a survey carried out by the research group Religions, Discrimination and Racism in School, based in Lyon.
"The teachers know when to defuse certain situations, for example when a student says 'you can't talk about Islam because you are not a Muslim'," said Sebastien Urbanski, a member of the group.
When confrontations do occur in the classroom, they are often not based on religious teaching, but events such as the Holocaust and the Israeli-Palestine conflict, said Bianco.
Teachers must field challenges from pupils of various faiths on religious questions.
For example, "some Christian circles contest the teachings of Darwin," Bianco said.
"It is quite an art to be able to teach students that what you hear in your house is not superior, it's just a different opinion. The teachers give the students what they need to form their own personal view," he added.
"It's that distinction between knowing and believing that is not easy."
Civic education
As well as religious teaching, the French education system has also put an emphasis on moral and civic education in schools since extremist attacks in 2015, in a bid to promote republican values and respect for freedom of speech and opinion.
But with just half an hour dedicated to it each week, there is not enough time to make an impact, said Eric Favey, of the Teaching League, a secular education association.
"In a society that is seeking to unite, if we ignore all of this, we are going to continue to wake up to bad news."
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