Depending on income students can get free or reduced price lunches. No there is no universal free lunch program in the US. Not sure how they do it now but it used to work just like food stamps, kids in the program had membership cards that got punched when they got lunch while everyone else paid money.
Many schools issue a card or PIN that is used to buy lunches. You can't tell who is getting assistance and who isn't, unlike the old days where the kids on assistance used punch cards and everyone else paid cash.
When I was in school we used cards but you could still tell who was on assistance because they got a different/worse lunch than everybody else.
I don't know what the statistics are. It's "news" though.