There was a kid, a tough and strong kid, who's grandpa died in a war, I don't know which. Anyways, when he saw that I remained seated for the national anthem, he told me to get up or he'd kick my ass. I told him bluntly that even if he'd threaten me with death, it still be seating. I also said that he should be quiet as he was, himself, lacking respect for the national anthem by talking while it played.
Somewhere during class, he told me that his grampa died at war. I told him that his grampa died for freedom, which included my freedom not to stand during the national anthem.
He said he's still beat me up anyways. So, I spent the next two weeks running away from him after classes until he finally let it go.
As for the teachers, they weren't all nice, but they usually didn't push it as word got around that some teachers had been suspended for lacking respect in this regard.