A long time ago, in Britain, the Union Jack was appropriated by racists and neoNazis. The usage of it was often as a provocation, most especially against Pakistani, Sikh, Indian, and Carribean immigrants. “There ain’t no black in the Union Jack” was something skinhead thugs chanted in the streets.
Because the flag represents so much to us in the United States, because it is something that we do, indeed, rally around, it is a symbol with quite a bit of power. I feel speech in school is a tricky-ass issue, but I feel that any sort of common decency says that the usage of the American flag on Cinco De Mayo by a group of individuals is meant, clearly, as an insult, a provocation, or a foolish, foolish defense of their perception of America.