> The American notion of "separation of church and state" refers to pluralism, not secularism

It's both. Agnostic or atheistic belief is one of the many faiths understood to be supported and protected by the separation of church and state. If a person can have faith in one god or many gods, they can have faith in none. It isn't French-style, but in a country where, for example, interracial and same-sex marriage is legal (in direct contradiction to the teachings of several religions), there are times where the state must step in to protect its citizens from having their rights stripped by organizations acting as a formal or informal arm of religion, so it can sometimes look like a state-sponsored secular faith.

The question is complicated and has gone the other way too; some religious ceremonies of native American cultures involve the use of psychedelics that are federally banned. Most Christian churches' celebrations of their covenant with God involve the imbibing of alcohol by people under the age of 21. In some cases, federal lawsuits have been involved to determine when the state is overstepping its authority to impose behavioral constraints on people.

> Our laws require equal rights and equal treatment in various contexts: government services, employment, etc. They don't say anything about "value" because in a pluralistic society people have different values.

In my mind, dignity and value are synonymous and the law definitely protects the dignity of those families. I'm unaware of a non-synonymous way to treat those terms, but if there is one I agree with your take on the law.

> "The States Parties to the present Covenant undertake to have respect for the liberty of parents and, when applicable, legal guardians to ensure the religious and moral education of their children in conformity with their own convictions"

The United States has never recognized the UN's full authority over its own laws or the interpretation of same. In the case of that quote, it's sourced from the ICCPR. The ICCPR was adopted by the United States with a declaration and proviso that fundamentally give it no force of authority here (https://h2o.law.harvard.edu/text_blocks/28885). In other words, the ICCPR is, at best, a declaration of morality and has no force of law, so if the US shares no morality or culture, you can't use it in this context to make an argument about how the US should act.

... which highlights the problem with the logic that the US shares no morality. Of course it does. We figure out what our shared morality is via the process of democratic election, debate, and construction of our law. That law reflects the shared morality of its people, which grows from our separate but overlapping faiths.

> whose culture?

The culture decided upon through the interaction of the school board and the election of the board by the people. With an overlap of the federal Department of Education. It's a complicated process, but it's there. There's definitely a culture to a school; without one, one cannot explain any proviso of any student handbook.

> pluralism requires respecting those boundaries

It absolutely does, and if a kid came home crying because their teacher told them they were living in sin with two fathers at home, that would be a major breach of trust on the part of the educator and we already have processes in place for dealing with that. But if the kid comes home crying because one of their classmates has two dads, the teacher said that's okay and the parents had told that kid it isn't? Then we have a clash of religion and the consensus culture of the country, and the school is correct in terms of its duties to teach students how to live in this country. For the law is clear, and the law is a reflection of the consensus morality. In this place, in the public square, in the streets, workplaces, and halls of government, it is okay to have two dads. This does not impinge on what one's heart says is okay.

My faith has an old saying, passed down from its prophet, which sums up this dichotomy nicely: "Give to Caesar what is Caesar's."