...the study shows that private food service is associated with a reduction of 1 percent to 3 percent in scores on the Michigan Educational Assessment Program (MEAP tests for grades 3-9), after controlling for affluence, school resources and student traits.This is especially true for students in grades 3-5 and with the English, reading and writing tests. The culprit? Private food services tend to serve more high-fat and high-sugar foods on their a la carte menu.
Schools often choose to contract with private food service companies to provide meals, hoping to save money that can then be directed to classroom education, but Zullo's study found that they don't save much at all. He also discovered that schools with privatized food service have an average of 1.1 student more per classroom. There are undoubtedly other factors at play and it's not a direct causal relationship, but as Zullo says, ".... the results do not indicate that privatizing food services liberates resources for the classroom."
Food for thought: privatizing school lunches may impair learning. University of Michigan News Service