"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances"
Say two parents of two students hold differing view. One is an Atheist, who says there is no god, and the other is a theist who says there is a God. Now if the government teacher says there is a God, the 1st Amendment right of the Atheist has been abridged, and if the government teacher says there is no god, the 1st Amendment right of the theist has been abridged.
The only way to uphold the 1st Amendment is by school vouchers, where the parent chose to put their kid in a school teaching atheism or teaching theism.
However, that is too deep for SCOTUS.