How do Catholic schools deal with same-sex "married" parents?

[HomeschoolDad]

When we enrolled our son, we had to submit a copy of our divorce decree with custody details.

That was the reason. The school needed to know who had custody, and what were the circumstances of custody and visitation, so as not to end up inadvertently doing something that would run counter to the divorce decree (e.g., a parent without custody coming to the school office and demanding that the child be dismissed to them at the end of the day, with the school being none the wiser unless they had the decree on file).

Whatever they require or don’t require, at an age-appropriate point in the curriculum, the Church’s teaching on marriage, in its entirety — including the illicitness of contraception, the invalidity of marriage after divorce without benefit of annulment, and the sinfulness of “gay marriage” — needs to be clearly taught and explained.

A Catholic school that does not teach what the Church teaches on these matters, does not deserve to be called a Catholic school. I realize your situation (in France, is that correct?) may be different, perhaps hindered by the state (laïcite). In the US, Catholic schools are entirely independent of the state, and may teach whatever they like.

That’s different. Your child (as mine) is homeschooled, so we are more free as parents to speak of what we want. That’s the advantage of homeschooling.

I teach the traditional Catholic Baltimore Catechism as well as using other resources.