I don't understand this POV at all. This wasn't people spitting on Vietnam vets returning home, in most cases it was a conscious decision made to try and bring issue to a REAL problem, the police brutality that happens in this country. No one was disrespecting soldiers or the flag in any way, it was kneeling during the anthem written by a man who openly was anti-abolition and went out of his way to prosecute people in some cases for distributing abolitionist materials. The greatest way America sets examples for the world is freedom, and people exercising said freedom is the greatest expression of American ideals there is to be made.
How many soldiers, career soldiers have you talked to about the issue? The majority I've spoken with feel like there was likely a better stage to protest, but most were proud that they'd fought ton give people the freedom to speak their minds and protest peacefully. Some of its generational, but some of it isn't.
I think if a person has been on the end of a situation where the police brutalized someone or witnessed some of the acts committed, they'd be more inclined to realize its a serious issue. High school kids can't fix the solution by themselves, but if they did it to make a political statement and not just to stand out, good for them for at least being socially aware.