Oh, please. If that was truly the case, the two who remained without changing would have been sent home, per the assistant principal's empty threat.
Look, there may have been -- indeed, on some level almost certainly was -- some undercurrent of needless provocation in the boys' culture-warrior antics. So what? There's nothing violent or incendiary about wearing a t-shirt with an American flag on it; there was no nasty anti-Latino sentiment scrawled next to the flag. They chose an awfully convenient time to demonstrate their patriotism, sure, but in and of itself, that's not a crime, nor even a suspendable offense.
Of course, we are talking about the public school system, which is not meant to educate or even indoctrinate, so much as housebreak, to encourage mindless conformity, whether through anodyne "cultural awareness" outlets throughout the school year, or some other expression of collective solidarity on this or that meaningless issue. As long as kids come out the other end as docile employees and consumers, lifelong receivers of baldfaced lies from their superiors, everything is jake. The second they pose an inconvenient question, or show any independent thought, instead of trying to talk with them, bureaucratic feathers get ruffled, and the warden drops the hammer.
Well, that's awful large of them, but get this -- it is America, and not Mexico, and while all cultures should feel free to express themselves on their important occasions, it's not the sharpest tack to take, to act like you're doing everyone a favor by holding back on the Fourth of July.
This is a very silly argument all the way around, the way people get butt-hurt over symbols and colors, especially over an occasion where a substantial portion of participants probably cannot tell you what is being commemorated. John Robb would probably characterize this as collective expressions of primary (tribal) loyalties, which in the continuing regression of defensible extended (belief and alignment with government and society) loyalties, makes sense. The more immediate effect is that this gives reactionary cultural "conservatives" more unnecessary fuel for their colorful assumptions.