School cantines vary enormously in that some are run by proper cooks who make almost everything fromscratch (and willvary in how good they are as cooks) and others have bought into the central cantine + reheat at school overhead-cutting system.
Examples: My children's école primaire in the village had an excellent cook (I turned up one day with forgotten sports kit & she was making the pastry for the tarte au chèvre) but she retired and as she was a mairie employee she has been replaced by someone who is nice enough (& has the food safety etc qualifications) but is NOT a good cook.
In primaire in town my daughter's not very imaginative meals come from a central industrial kitchen and are then reheated and unsurprisingly they are not as good. They were probably better first time around.
My children in collège & lycée are lucky enough to have on-site proper cooks and have very good meals and my colleagues eat in the lycée canteen saying that while it is of course collectivité food, it is very good for what it is. The kitchens at the lycée make meals for 1500 people with a choice of 2 or 3 entrées, 2 main courses, cheese and pudding and fruit.
In my own cité scolaire most of my colleagues prefer to bring their own lunch but mainly for time-saving reasons as we get an hour for lunch and there is always stuff to do, people to see.
I think the attitudes to children not eating and being punished must depend enormously on the schooland I think it is écoles which are concerned, not collège or lycée - in primaire the meals are supervised by mairie employees who are often of a certain age and have corresponding attitudes, some understanding and kind, others not. Mairies especially in small villages take whoever they can to do a job with anti-social hours for very little money.