The change actually started in the mid 1960s when busing to achieve racial balance started. Prior to that kids walked home for lunch from neighborhood elementary schools. Only the junior and senior high schools had cafeterias that served hot lunches. The neighborhood schools did not have commercial kitchens, so prepacked lunches in disposable individual containers came on a truck each day from the central commissary. in rolling warming units, that could be plugged in to stay hot and all they needed in the grade schools was a refrigerator for the milk and folding tables with benched to set up in the gym.
By 1980 all new schools were constructed with kitchens for heating and serving. They had warming ovens and refrigerators. NO stove-tops, no pots and pans, no serving utensils, NO dishwashers and NO cooks
This system is far less expensive than cooking on site in terms of equipment cost and personnel. A 4 hour a day lunchroom lady costs about $10 per hour and gets no benefits working 19 hours per week. A cook would be paid about $22 plus benefits..add in the cost of equipment and utilities and the price of lunch would be prohibitive.
Our town uses this system. They sell the complete Type A hot lunch (Main, starch, fruit, veg and milk or juice) for $3 (breaking even). If they went back to the old ways, cost analysis (assuming a 25 year life to equipment) shows they would have to charge $4.75 to break even.
Our local day school changed from cooking on site to having a local kosher caterer (who also supplies meals on wheels and runs the kitchens at the Home for the Elderly) run the program back about 1995. It also was a huge cost savings. A professional operation turning out 3000 meals a day was far more efficient than a single cook in the day school kitchen. We also got nutritionally balanced meals for the kids. No more twice a week fried matzo with powered bug juice from after Pesach through June.