What I don't get about lunch in school is where this assumption comes from that it has to be a warm and complete meal, ideally with starter and dessert.

I am really wondering about that. School is more like an office job than a construction site or other heavy work and thus would not a sandwich and a salad be more appropriate? It would be much easier to offer good quality this way, especially when it comes to fruits and vegetables. The stuff you get when cooking with a limited budget for hundreds of people is seldom good or healthy.