Separation of Church and State or Public Schools: Pick One
This suggests a more general point—is the existence of a public school system consistent with a serious commitment to the separation of church and state?
I think the answer is that it is not. While teaching a fundamentalist version of the origin of life is indeed taking a side in a religious dispute, teaching a conventional account of biology and geology is is also taking a side in that dispute, just the opposite side. I do not see how I can honestly tell a fundamentalist that it is a violation of the separation of church and state to teach children that his religious beliefs are true but not a violation to teach children that they are false.
The conventional view is, in this case, the one I believe is true. But then, if they were teaching creationism, they would be taking the side he believes is true. So what purports to be separation of church and state ends up as the opposite—the state supporting a particular view of religious questions. That comes pretty close to the established church that the First Amendment explicitly forbids.
Of course, these are not only religious questions, they are also scientific questions. But then, most religious questions are also scientific, or historical, or philosophical, questions. If the rule is that the state can teach whatever it believes is true provided that here is some basis for that belief other than religion, that leaves the state free to teach the truth or falsity of pretty nearly every religion. The doctrine of separation of church and state then becomes the doctrine that one can only teach the truth, which sounds fine as rhetoric but has some practical difficulties in a world where different people have different views of what the truth is.
So far, I have considered a case where the school teaches what I believe is true. In the real world there is no such limitation, as the quoted comment with which I started this suggests. When schools teach children that they have an obligation to take care of Mother Earth they are teaching religion, whether or not they put it in an explicitly religious form; religions are not limited to beliefs about gods. And I find it hard to draw any sharp line between religions and secular ideologies such as Marxism or libertarianism.
Eliminate all content that is in a broad sense religious and there is nothing left. Even eliminate all content that is religious in a narrow sense, where that includes claims that religions are false as well as claims that they are true, and there is not a whole lot left.
In practice, the application of separation of church and state in the American public schools usually comes down to not teaching what most of those concerned see as something that one would believe only for religious reasons. A century or more ago, that mostly meant that teaching Christianity was fine, since practically everyone took it for granted that Christianity was true. Today, insofar as matters are decided at the local level, it means that teaching things that the locals almost all agree with are fine—which can be Christian fundamentalism in some places and environmentalism and left-wing politics in others.
Problems arise when there is a conflict either between local and national views or between the views of the local parents and the views of the teachers and/or administrators running the schools. It is only at that point that what one group sees as obvious truth gets attacked by another as teaching religion.