Quote from CaptainObvious:
It just isn't relevant. If I wanted to show up at a Mexican festival/party/rally/protest/whatever, on American soil, waving the flag and singing God Bless America, I have the right to do that. I might get shot, but that ain't the point. It's American soil! It's an American flag and I'll show it where I damn well please, least on my own soil.
Edit! In re-reading my post, "I might get shot, but that ain't the point". Well, that's the whole point. If I can't show my flag in my own country without fear of discipline or harm, something has gone very, very wrong in this country.
You could, at that party, sing God Bless America with a certain tone of voice, making eye contact while using certain expressions, both of which could antagonize that other person. Hell, with the right tone of voice you could anger anyone, singing that song. And that's the idea with this incident, it's not the flag per se, it no doubt has something to do with five boys all wearing flags on their t-shirts AND bandanas, on a day where their school is celebrating an immigrant group's holiday, and all having lunch together. I don't think that's a coincidence.