These kids aren’t getting fat from “school lunches”. If anything, it’s all the junk they eat when they get home.
It’s obliging parents, not knowing how to say, or enforce, the word “no”.
In my day we walked to school. If we went back to neighborhood schools (schools within a walking distance of 1 mile for elementary, 2 miles for middle school, and 3 miles for high school) there wouldn’t be a fatty in the bunch. In the old days, there always seemed to be one kid in each class who was considered fat and who received the elephant Valentine cards but no more than one. Additionally, we didn’t have to carry much more than a skinny textbook or two plus our lunch which we made ourselves and brought from home in a brown paper bag. We bought milk for a nickel at lunchtime. With the backpacks that kids have to carry these days everyone would be thin by Christmas at least.