The ban, passed in July following a campaign pledge made by French President Emmanuel Macron, will affect elementary and junior high schools across the country as they return from the summer break.
The new law, which went into effect on August 5, bans all types of cellphones, as well as tablets and smartwatches.
While a ban on cellphones during class hours was already in place since 2010, the new law extends to breaks and mealtimes.
Schools are free to choose themselves if they implement the ban for students over 15. There are also some exceptions to the prohibition, such as for students with disabilities.
Under the new law, students have to turn their phones off during the day or put them in lockers. Schools will independently deal with the logistics of how students will be kept away from their phones.
The law was introduced amid fears that students were becoming too dependent on and distracted by their smartphones.
Education Minister Jean-Michel Blanquer in June hailed the legislation as "a law for the 21st century," and said it would improve discipline among France's 12 million schoolchildren.