In Busch v. Marple Newtown School District nobody is being denied the privlidge to live their own lives. The first ammendment guarantees free speech but it does not guarantee a captive audience, which is what the court ruled a class of kindergartners is.
As to the idea that people should keep their kids out of public schools if they don’t want their kids learning about other religions, 1) their is a difference between learning about a religion and being subjected to proselytizing and 2) the public schools belong to everyone, not just the Christians.