Since nutrition standards were strengthened in 2010, eating at school provides many students with healthier food than is available cheaply elsewhere. Plus, reducing stigma increases the number of kids getting fed.
My wife works at an elementary school, and a fair number of the kids depend on those school lunches as the only food they might eat that day. The school is in a mixed and low income neighborhood, and for some of the families this is how their kids get most of their meals.