Allow students to refrain from singing religious text contrary to their beliefs in public school choir.

The documents stating Separation of Church and State are meant to protect students with a variety of beliefs from having another religion forced upon them, and the First Amendment to to U.S. Constitution not only grants freedom of speech, but freedom to not speak, and freedom to petition.
Public schools, however, do not seem to understand that they are not allowed to pick and choose which laws apply to their students (Tinker vs. Des Moines case of 1969- "Students do not leave their rights at the schoolhouse door.")
Michigan School Vocal Music Association (MSVMA) states in their policy that "performance of religious texts have nothing to do with the First Amendment to the Constitution and the doctrine which advocates separation of Church and State."
I believe they're wrong. Please help me.