Very marginal if at at all. Believe me especially by middle school and older, everyone knows who is poor, who is rich, who is a slut, who is religious, who is a jock, etc. And there are other ways, e.g. everyone has a swipe card or number code to "pay" for their lunch and you never know who's on the free lunch program or not.

I've never heard of a school giving free lunch by default, though. Maybe in areas where 99% of the kids would qualify anyway?