The Star Spangled Banner became official as the national anthem in 1931, but after doing some searching, and finding a lot on when it became popular at sporting events, I can't find any mention of when it became common to sing it in public schools. Does it have anything to do with America's red scare and the Cold War?
Can you show some evidence that it IS common to sing in public schools, because the only time I've ever heard it in school was at graduation or a sporting event.