Even if perfectly legal to offer, it is entirely inappropriate for an American public school to offer the pledge of allegiance to a foreign nation. Period. End of story. I would be raising holy heck, speaking loudly to anyone who would listen, and raising Cain at every public forum until the practice stopped and someone got their tail end in a losing offering up such a thing.
Unless this is an embassy school for the children of diplomats from Mexico, the school has no business promoting allegiance to a foreign power. If they want to pledge allegiance to Mexico they can do it on their own time or they can head south and go to school there.