I'll make this simple.

It doesn't matter where you came from now that you are here. If you're here legitimately, you're an American.

You're either an American or you're not. "Mexican-American" is bullshit. If you don't have dual-citizenship (which very few people do) you are not a "Mexican-American", you're an American.

You absolutely don't get to complain about students wearing shirts with an American flag on them, Cinco de Mayo or not. You particularly don't try to force them to be sent home from school because you want to feign being offended. If you want to celebrate a "Mexican" holiday without anything of this country being involved, do it in Mexico. ...otherwise you're just another selfish asshole trying to force other people do to things your way.

Doing such a thing completely disrespects why this country was created in the first place.

Acclimate or GTFO. It's that simple.

Students sent home for wearing American flag shirts