They don't. In my experience that would be exceedingly rare and I have never heard of it happening.
What schools do promote is the idea that every child is welcome in school and should not be discriminated against or bullied regardless of their race, looks, ethnicity, culture, religion, abilities, and yes, sexuality. And that is exactly what they SHOULD be doing. In fact that is the actual LAW.
That is different from promoting any specific one of those things, like a specific religion or specific sexuality or favoring specific ethnic customs.
So schools should say that all students are WELCOME in school regardless of whether they are Catholic, Muslim, Anabaptist, Hindu, or none of the above But schools should not be PROMOTING any of those specific religions over any other.
Likewise, should should say that all students are WELCOME in school regardless of whether they are straight, gay, trans, or non-binary which is just the new lingo for what used to be called things like tomboys. But schools should not be PROMOTING any of those things or advocating that students follow any specific sexuality. The standard should be no different.
Are there teachers who violate either of those two standards? I'm sure there are fringe cases were you can find either or both. Because there are millions of teachers in the US. But in my experience, neither of those things generally happens. Nor should it.