I'm not your friend Gaylord. The fact that you can't see wearing the flag on the Mexican holiday as offensive is actually funny to me.
First, Cinco de Mayo is not, repeat not, a Mexican holiday. Calling Cinco de Mayo a Mexican holiday is a bit like calling March 2nd an American holiday. (Since you are obviously lacking in a firm grounding in history I will point out that March 2nd is the day Texas declared independence from Mexico.)
Second, Cinco de Mayo was the first win in a long war that was ultimately won only with the help of the US Army. I think that makes the US flag at least as important to the celebration of Cinco de Mayo as the Mexican one.
Third, even if it is offensive, so what. People have a right to be offensive, if they didn't, someone would have killed you long ago. If my children were in school I would find it offensive that they were being forced to celebrate a holiday that is promoted with the sole intent of selling more beer to a minority group.