That is ridiculous. Cinco de Mayo is not even widely celebrated as a holiday in Mexico. I have encountered many who believe it is Mexican independence day, which it is NOT. (it's in September). It is a celebration of a victory against the French in a battle over unpaid debts in the 1850's. (The French later won and occupied Mexico for some time). This "holiday" is basically an excuse for Americans to drink.
Now, on a more important note, EVERY day is a day that is appropriate to display the U.S. flag. Even if Cinco de Mayo was an actual important holiday to Mexicans, that does not override the display of the U.S. flag! The fact that this school "official" has decided to take this action in response to what is, in effect, not even an important holiday, just makes this that much more frustrating.