The announcement follows an agriculture and food law (loi Egalim) that was passed back in 2018 - and states all schools need to offer at least one lunch every week that doesn't contain meat or fish.
According to Local France, Greenpeace spokesman Laure Ducos said: "There has been very little information circulated from the Ministry and there has been no decree.
"There are therefore some cities that believe that it is not mandatory because there has not been a decree, but that is not true: the law has passed and it is, therefore, important to recall these obligations."
Rodrigo Arenas, president of the Fédération des Conseils de Parents d'Elèves - who has worked alongside Greenpeace - added: "It is also the school's role to teach students to eat less meat for their health."
Earlier this year, Celebrity chef Jamie Oliver said it would 'brilliant' if schools went vegetarian during an interview with The Herald Scotland, in which he promoted his new cookery show Meat-Free Meals.
"Generally, what a child and what a family needs is the same as what the planet needs - more veg, more nuts, more seeds, more legumes," the star added.
"If I had a magic wand, I'd love to go to David Attenborough and say, 'Can we do a show called My Health, My Planet?' Because I think that's the conversation now."