If the school mandates the song, the school violates the First Amemendment. If the graduating class voluntarily sings the song, it is protected by the First Amendment.
The U.S. Constitution is a wonderful document.
Why did it take over 140 years for judges to see that is the case?
According to the consitution and first amendment, schools can teach or mandate anything they want. Or they could until liberals took over the Supreme Court and wrote new law from the bench.
If you want to censor religion, pass a law or repeal the First Amendment. Don't play liberal mind rot and pretend there is some consitutional basis for your opinion. Public schools were originally set up to push religion. Massachusetts enacted the first mandatory attendance law in the nation because they wanted to bring Irish Catholics into a Protestant culture.
The Klu Klux Klan got a law passed in Oregon in the 1930s essentially banning private schools because they did not want Catholics to set up their own schools, they wanted them in Protestant government schools.