I personally say "always a no no." Obviously, it becomes impossible to control what high schoolers do at every minute of the day, but as a mother of young children, I would want teachers to be echoing MY lessonsnotto ask others for food. In your case, it seems more (or less really) is going on at home and these kids are hungry.
I never offer students food, and discourage them from asking. It sets a bad precedent in the classroom for what could later be construed as favoritism, not to mention the liability that could come with giving a student something he's allergic to... I don't know, the possibilities are endless and it is a door I choose not to open.
At some point, the parents need to be held accountable, but if you are providing food, or encouraging students to ask others, you are only enabling the bigger problem.