That's a good question. Probably one has to go through the hoops and become a citizen of Mexico. Many Americans can teach sort of like "wetbacks" or undocumented workers and are paid good money in cash. Working for the PUBLIC schools has a lot of labor union built in rules that would make it difficult for an American citizen to teach for pay in Mexico.
You miss my point. I'm not inquiring about employment of Americans, the question is about the subject matter: Is it legal for anyone to teach English as a subject in Mexican public schools?In a country which reportedly publishes information for domestic consumption on how to evade US immigration restrictions, anyone would suppose that the schools would also teach English. But it seems that not that many Mexican immigrants do know English when they arrive - and it's a significant handicap to them.
Even for those who do not intend to emmigrate to the north, English is a valuable language not only for tourism but for international commerce generally. Do Mexican public schools teach the subject?