I could easily write a thesis on this topic but I will limit myself on this topic.
It is not difficult to prepare a nutritious meal for 900 people at a very reasonable cost. I spent ten years of my life operating a commercial kitchen in a healthcare setting serving 3-4 meals per day, 365 days a year etc.
However, several factors in the school setting make it a lot more difficult.
1) In many cases, you are forced to use inferior products which are available from the USDA. When I worked at a public hospital, the hospital received commodity products from the federal government as the hospital served an indigent population. The quality of food was miserable.
2) Many of the public school systems bid out all of their purchases. That is good in that it reduces costs. However, in the bidding process, the low bidder is the low bidder because the food is complete garbage. The purchasing authority within the school system does not enforce the quality standards.
When I talk about quality, we would pull out over 1# of pits and stems from 6 #10 cans of fruit cocktail. if I legally could, I would have bought from places like Sysco or Gordon's Food Service but that was not an option.
3) I have never seen a school with a $5+ per pupil budget. WOW! You could do a lot with HALF of that.
4) As most schools serve a small number of meals, the great move has been to go to central commissaries where the food is prepared and distributed. That can be done right. However, if it is not done right, the food quality is pretty poor.
5) Personally, I did not attend a school with a cafeteria until junior high. And the public school lunches were pretty good - good enough that I put on 15# the first year. And this was REAL food not the pizza, hamburger, fish sandwich, chili mac - that you see today. And 50-60% of the food was thrown away.
What are you going to do, force-feed good food to students? Or have a lunch room monitor make sure that everyone eats their peas? Dietary choices start at home with the parents.