In the US, this would be a problem because a lot of kids depend on school-provided meals as their only or primary food source, and vegan diets don't work for everyone. (Crohn's Disease is the example we hear about the most, but there must be others.) There would be kids who needed omni meals for medical reasons.
I don't know if the same is true in France or not.
Here in Brooklyn, our borough presisent is vegan, and our schools have Meatless Mondays.
I think it's a great idea, as long as they're realistic about exceptions and accomodations - make meatless the norm, but provide an opt-in option for omni meals if needed.