Apparently, they've forgotten this U.S. Supreme Court case's majority opinion:
"The fundamental theory of liberty upon which all governments in this Union repose excludes any general power of the State to standardize its children by forcing them to accept instruction from public teachers only. The child is not the mere creature of the State; those who nurture him and direct his destiny have the right coupled with the high duty, to recognize, and prepare him for additional duties."
—U.S. Supreme Court Decision in the Oregon School Case, June 1, 1925.