As a sibling comment noted, the National School Lunch Program started in 1946. [1]
The School Breakfast Program started in the 1960s/70s. [2]
More recently, in 2010, the Healthy, Hunger-Free Kids Act was passed which enabled the Community Eligibility Provision which lets low income area schools offer free food to all kids without individual application/qualification. [3][4]
Various schools I went to in the 90s/00s had some kids bringing their lunch, some kids paying the school for lunch, and some getting free lunch (because they were poor enough to qualify).