I went to school during the same time period and we'd never heard of a school cafeteria. Our Moms (mine was a single working Mom) packed our brown-bag lunches daily. I remember Sockeye salmon sandwiches (the canned was cheap then), tuna, peanut butter & jelly, always with an apple or orange for "dessert." We had fat kids and thin kids and in-between kids but in those days we realized body types differ a lot based on genetics. This whole "obesity" binge is just more meddling from Lefties who want to run everyone's lives.
We brown bagged a lot in grammar school. I don’t recall teachers or other school officials saying anything to the few overweight kids but they were out there at recess and PE classes with the rest of us.