Students are required to say the Pledge of Allegiance at the beginning of every day (though few take it seriously or even stop to consider the words) and at the start of all sporting events. Some schools do this once a week, while others are more diligent in their practice of the law.
The Star Spangled Banner is not required by any law that I have ever heard of, at least at the national level, or at the Washington State level. I think some people have this misconception because it is played before all professional sporting events.
I think maybe your question is should it be the law to sing "The Star Spangled Banner" not is it the law. It is true that this song is less controversial than the Pledge of Allegiance, but the idea is the same. Many countries do try to indoctrinate students politically from a young age. While some schools in America do the same thing, it is not, nor should it be, legally required that they do so.
I've taught in 5 schools, both public and private, and none of them has ever required a student to sing anything--"Star-Spangled Banner" or anything else. I can't even imagine the scenario in which such a song would be sung during the school day. Sporting events are another matter, and those are certainly not compulsory singing events for anyone in attendance. I love the song and actually get choked up as I sing and salute the flag; however, I would never think it right to force someone to sing it in any forum under any conditions. "Required" is a funny word, and its implications range form expected to legal. I'm wondering if it's more on the expected end of the spectrum. I cetainly hope it is. It's not right, and it's certainly not legal.
Although there may be some school districts that mandate that the United States' national anthem, "The Star Spangled Banner," be played during school or school-affiliated events, there is no Federal law that requires the performance of this song in public schools. None of the schools in which I have taught (during a 25 year career) required that the national anthem be played during school hours. Francis Scott Key's song is most often heard at the beginning of sporting events (including many school functions), just as it is played before most professional athletic events in the U.S. As for it being required to be sung by students, I imagine that very few schools would try to maintain or enforce this rule. The song is very difficult to sing in the first place. Generally, audiences do stand during the playing of the song; hats are removed; and many people choose to place their hand over their heart. Doing so shows a patriotic reverence for the occasion, but that is only a general rule and not a requirement.
I see that your child may receive a "zero" if he/she does not participate. I find this a highly preposterous proposal obviously being implemented by a patriotic and probably very conservative administrator who is pushing his own personal beliefs upon the entire school. The only way I could see this rule being implemented would be if it was part of a music or choral class; refusing to participate then might warrant a grade reduction--but a zero!?. I feel your pain, and I hope you are able to work this problem out if you feel it is a breach of your child's personal freedom of choice. Good luck.