First of all, this debate isn't about "standing for the flag." It's about standing for the national anthem. They're not the same.
Secondly, the national anthem (like the flag) is a symbol; it has little meaning in and of itself. It stands for something. But... from the standpoint of American minority populations, what it often stands for is a history of systematic oppression. Black Lives Matter is a reaction to a resurgence of police violence against minorities - of STATE violence against minorities, of STATE-TOLERATED violence against minorities by bigoted individuals.
People protest, because our courts are looking the other way, failing to convict obvious murderers who happen to be police officers. People protest, because our Attorney General and our President see nothing wrong with this. People protest, because Donald Trump cannot bring himself to condemn Nazis who march with torches and guns, who threaten people, who kill... but can easily condemn a football player who dares to not stand for the national anthem.
Do you understand why your administrators sent you that memo? The Supreme Court concluded that requiring students to stand, and/or to recite the Pledge of Allegiance is in fact requiring - compelling - students to engage in political speech. Forced political speech is a characteristic of totalitarian societies, not of democracies. Demanding that people must stand to "honor" a flag, or an anthem, or any symbol of a government with which they disagree, is a characteristic of totalitarians - of fascists - NOT of free people in a free society.
By the way, the Supreme Court decision that determined kids could not be compelled to stand and recite the Pledge of Allegiance? It was handed down in 1943, when of course the "left" was running wild in the United States - was in fact in the process of crushing certain upstanding right-wing governments in Europe and Asia.
But they are not "turning their backs" on American taxpayers. They object to state violence against minority individuals. Are you arguing that taxpayers in general support (or participate in) murderous violence against minorities; that taxpayers in general are hateful bigots? That taxpayers in general share Jeff Sessions' views on how to treat African Americans?
These athletes are protesting against failings in our system. They're not turning their backs on America - they're trying to make it a better society, by drawing necessary attention to our current failings. Yes, that makes people uncomfortable. But we can never overcome our failings, about race in particular, if we just want to ignore them. And "let them protest, but not on my time" is a cry to be left alone, to allow you to ignore these real problems.
Furthermore... Why do we play the national anthem before sporting events? We don't do this before dramatic performances, or concerts, or even sessions of Congress. What's the deal with ball games? Do we really need - do we even benefit from - the militarism and jingoism that is becoming common at sporting events?
What irritates me - and more and more, disturbs and disgusts me - is that too many Americans associate what is good and important about America with the military, with wars, with violence. We routinely - no, constantly - pay homage to current military, to veterans, to "first responders" (which mostly got thrown in there a couple of years ago as a backlash to growing questions about out-of-control police). We don't routinely honor teachers, doctors, nurses, assorted others who work in service to others. We don't recognize people who work in occupations that are more dangerous than being a police officer or a soldier - and yes, there are at least several, from miners to fishermen, who take greater risks than cops, to provide you with food and fuel...but we don't idolize them; why not?
We pretend that our veterans, who have followed orders and gone off to fight foreign wars (to their credit), have somehow "preserved our freedom" - as if our conflicts of the past 70 years have somehow been in response to serious threats to that freedom. This is rubbish. Nazi Germany had aspirations of world fascist domination, and was rightly considered a threat to the freedom of the United States. But Korea? Vietnam? Assorted mini-states in Latin America? Serbia? Iraq? Afghanistan? Libya? Syria? Come on - no thinking person believes any of these wars was about "preserving our freedom." Preserving our power, yes; preserving our economic interests, yes. But these are different - and they're not so noble as "preserving our freedom," which of course is why we don't talk about them.
You earlier posted:
Trump is not always loquacious in his use of words but he speaks to the heart of many Americans and that is why we like him.
I don't know if "loquacious" was quite what you wanted there... but as SF noted, it's disturbing to think that Donald Trump "speaks to the heart" of any Americans. This man promotes division; he mocks the disabled, he denigrates whole classes of people on the basis of their nationality, he's openly abusive of women, he (occasionally) fakes some level of belief in some indeterminate brand of Christianity which he clearly does not take seriously. He has pretty evidently spent much of his "business career" doing shady things with other peoples' money. He cheats those who work for him. He is loyal to nobody but himself.
This man "speaks to the heart of many Americans"? I sincerely hope you are wrong about this...because if this is what is the hearts of many Americans, the American experiment is a failure. This man is despicable; he rejects the core philosophy of the Founding Fathers; he rejects the core tenets of essentially all the world's great religions; he values nothing but himself.