So, not speaking directly to lunch, but to the food per day in a pub school:
My kids are both in elementary. The school serves every single kid breakfast. The kids are supposed to choose the day before whether they will take breakfast or not, but even if the kid says no, they still serve them breakfast. The kids all are told that if you don't eat it, to save it for mid day snacks. The breakfast is something that won't go bad sitting in the open air for a few hours about 75% of the time, so it works for snacks. Then, the school always has three choices of lunch. Two warm and one cold with the cold usually being a chef's salad. The warm choices are very similar to the stuff they served when I was a kid but has been revamped ingredients wise for the new calorie requirements. For example, the rolls used to be buttery and delish. Now they are just reg rolls. The pizza used to be super greasy, now it's a different brand and isn't greasy. Also, the school has a rule that any kid can add as many extra veggies as they want to the normal lunch.
I have eaten with my kids at school numerous times and it is fine.
I get districts are all different, but the difference would have to be night and day for this to really be a thing.