The law does not allow one to discriminate upon certain grounds even if it is the most efficacious choice.
But that is not what you said. Let me requote you again:
My argument is that a perfectly neutral American accent is not absolutely necessary to teach children.
Your assertion is that an American accent is not absolutely necessary to teach children. Your argument makes no mention of moral or ethical implications, only that a neutral American accent is not absolutely necessary. I pointed out that the argument for it is based off what is most efficacious, not what is ethical or moral. You were not arguing the moral or ethical implications in your post, only what is absolutely necessary.
But, if you want to bring the moral and ethical side into it, what is the definition of absolutely necessary? I would say the efficiency of learning and providing the greatest amount of good to the greatest amount of people would qualify as "absolutely necessary." If your definition of "absolutely necessary" is "whatever works well enough to get by," then I would like to point out that is exactly the reason our education system is in such decline. It is exactly that line of reasoning that our public school system is in such sorry, pathetic shape.
No child left behind.
Cater to the lowest common denominator.
Teach to the test.
Etc...
We hamstring and neuter our education system so that we do not trample on the sensitive feelings of those that can't or won't keep up or make accommodations for the culture they have decided to be a part of. When I move to other countries (or even just travel) I make every effort to conform to local customs, traditions and even language. I do not (Yeah, I know, rare for an American) try to force my language or ideals on my host culture. I listen to how they speak and I try to imitate it as best I can. I very seriously dislike the "polite" people that will not correct me when I make a mistake - I want to be corrected, I want to understand and I want to be understood. No person in a foreign (to them) culture should strive for anything less. To not do so is not only rude but the height of arrogance.