It's one thing to wear the American flag on any other day. But to specifically wear it on Cinco de Mayo, a Mexican holiday, at a school where there is a large number of Hispanic students is incendiary and was, without a doubt, done on purpose to start "something" and to make a statement. I highly doubt if there were the same number of Irish students at the school would those children be as "patriotic" on St. Patrick's Day. They'd be wearing green, pinching people and getting just as intoxicated with the rest of them.
They have the right to wear the American flag on that day, just like I have the right to question their motives.