Inevitably, it's a political football.  There is no legal obligation to provide canteen meals at all at schools, nor any obligation that they should conform to any specific alimentary regime, religious or not. But there is nothing preventing schools from offering a halal option, and many do. But given that pork is by far the cheapest meat, and that school canteens have limited budgets, it's not an easy one for canteens to resolve.

From Le Monde :

Tous les observateurs reconnaîssent qu'il existe une hausse des demandes de menus spécifiques dans les établissements scolaires depuis une dizaine d'années. Globalement, la demande est passée, en un peu plus de trente ans, du "sans porc" au "sans viande", puis plus récemment au "halal". Ces nouvelles revendications émanent des parents de confession musulmane. "Les modérés demandent que leurs enfants ne mangent pas de viande ; les ultras, qu'ils mangent halal", disent les acteurs de terrain.

La problématique se pose moins pour la communauté juive et les repas casher, notamment parce que 30 % des enfants juifs - issus des familles les plus pratiquantes - sont scolarisés dans les établissements confessionnels.

All observers agree that there is an increase in requests for specific menus in schools over the last ten years. Overall, requests have shifted over the last thirty years, from "without pork" to "meatless" and more recently to "halal". These new claims come from Muslim parents. "The moderates ask that their children do not eat meat, the ultras, that they should eat halal," according to sources.

The problem is less acute for the Jewish community and kosher meals, especially because 30% of Jewish children - from the most devout families - are enrolled in religious schools.

_{It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II}