I think our main issue here is the "school" is taking a political role. They are teachers there for an education. Leave out the politics. This "shirt" has no reference then any shirt that is worn. If someone wore a Canadian flag shirt or a Mexican flag shirt they should have the same consequence then. On any day. We do not celebrate cinco de mayo officially here. Its another day in the calander. Same as July 4th in Mexico, Turkey, or Russia.
I forsee a lawsuit.
I will leave with this thought then I'm going back to building jigs.
When you become a citizen, pick your country. Doesn't mater. Your old citizenship takes second place. If you do not agree with this, then why did you move?