Checks by the French Interior Ministry on the number of Muslim children who skipped school last month to celebrate Eid Al Fitr drew criticism from trade unions and anti-racism groups on Monday.
The festival concludes Ramadan and is observed as a holiday in Muslim-majority countries.
This year, Eid Al Fitr fell on Friday, April 21, for most believers.
"An evaluation of the level of absenteeism recorded on the occasion of Eid Al Fitr" was ordered by France's Interior Ministry on Sunday.
The ministry "regularly studies the impact of some religious festivals on the workings of public services, and notably in the educational sector", junior minister Sonia Backes said in a statement.
In the city of Toulouse, police asked the heads of local schools to report the number of children absent on April 21, leading to accusations that authorities were creating a registry – which was denied by Ms Backes.
The country's biggest teachers' union, the FSU, said in a statement addressed to Interior Minister Gerald Darmanin on Monday that it "harshly denounces this operation".
"Attempting to create statistics by security forces on religious beliefs and their observance or not, above all in a school environment, goes against the basic principles of secularism and fundamental rights," the FSU said.
Eid Al Fitr preparations around the world - in pictures
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A man shops for sweets at a market before Eid Al Fitr in the port city of Sidon, Lebanon. Reuters -
A street vendor sells balloons in Sidon. Reuters -
Ministry of Religion employees in Aceh, Indonesia, observe the position of the Moon to determine when Eid Al Fitr will fall. EPA -
A vendor selling sweets as Palestinians prepare for the Eid Al Fitr holiday in Khan Younis, the southern Gaza Strip. Reuters -
Palestinian women make traditional cakes in Rafah, the southern Gaza Strip. Reuters -
A woman has her arm decorated with henna at a street market in the run-up to Eid Al Fitr, which marks the end Ramadan, in Mogadishu, Somalia. AP -
Henna designs are the order of the day in Mogadishu. AP -
Pastries on display at a market stall in the Ulus district of Ankara, ahead of the Eid Al Fitr holiday. AFP -
Many Muslims head back to their home towns and villages from bigger cities to spend Eid Al Fitr with family, such as this man and his children leaving Dhaka, Bangladesh. EPA -
The sunglasses are on as the packed train leaves Dhaka. EPA -
Passengers prepare to board a train in Surabaya, Indonesia, as people head home for the Eid holiday. AFP -
Excitement is building for Eid Al Fitr in Karachi. EPA -
A pet hotel attendant with a cat staying at the facility while its owners enjoy the Eid Al Fitr holiday in Banda Aceh, Indonesia. EPA -
A Pakistani craftsman works on women's embroidered clothes before Eid Al Fitr in Peshawar, Pakistan. EPA -
Embroidered cloth being stitched for Eid Al Fitr. EPA -
Passengers at a bus station in Banda Aceh start the journey home for the festivities. AFP -
Traffic at a toll booth as Muslims head home for Eid Al Fitr, known locally as Mudik, in Karawang, Indonesia. Reuters -
Iraqi women prepare traditional cookies in Mosul ahead of Eid Al Fitr. AFP -
The bakes are known as kleicha. AFP -
Women visit a market ahead of Eid Al Fitr in Karachi. AP -
A Palestinian girl prepares traditional date-filled cookies with her family in Rafah, in the southern Gaza Strip. AFP -
A Palestinian family prepares food for Eid Al Fitr. AFP -
A three-wheeled taxi, known as a 'bajaj', navigates its way through Tanah Abang market in Jakarta. AP -
A vendor waits for costumers at a market in Jakarta. AP -
A Palestinian worker prepares smoked fish at a factory in Rafah. AFP
The smaller CGT Education union called it a "scandalous and dangerous stigmatisation".
Using police to carry out the checks was "particularly shocking because it associates the observance of the Islamic religion to an issue of security", the anti-racism group SOS Racisme said.
France has a strict form of secularism that seeks to separate the state and its various branches from religion and religious bodies, while guaranteeing the freedom to worship to all.
Collecting information about ethnicity or religious beliefs is also generally prohibited in France under the country's anti-discrimination laws.
Owing to the country's Catholic heritage, major Christian festivals such as Christmas or Easter are observed as public holidays in France, and schools are closed.
