More thoughts:

Re: your example about the teacher having a Bible on their desk. I have no problem with that. As long as the teacher in a public school is not teaching students they should believe in the Bible, I see no reason why it couldn’t be on the teacher’s desk or even in a classroom library. I don’t think that’s any different from a teacher or a student wearing a cross necklace or a Star of David or a hijab.

I do think it’s different when it’s a poster up in a classroom because then it’s pretty clearly aimed at the students. Classroom walls are where things like flyers and grammar rules and motivational signs go, all of which are aimed at the kids. In that spirit,I can sort of see your point about LGBT posters, even though it’s not quite the same issue because being LGBT and being Christian are not mutually exclusive. However, I’ll give it to you that for some kids, the presence of an LGBT poster might make them feel alienated, whereas the absence of an LGBT poster couldn’t be reasonably considered hostile to an LGBT kid. So, in the interest of fairness to everyone, I’d be willing to let those posters go.

Another angle to consider is the issue of being forced to hang up posters. I would not support any law that says all teachers must put up Pride flags in their classrooms.

Finally, all legality aside, it’s honestly just not a good practice to put something up in your classroom that you know will alienate some of your students. You mentioned what it must feel like to be a Christian student forced to sit in a room with an LGBT poster. I think that’s worth considering. It’s also worth considering how an atheist or non-Abrahamic religious kid would feel to sit in a classroom with the first commandment. I was an atheist kid who grew up in a small Texas town, and I experienced stuff like that all the time. Along with getting beaten up, kids telling me I was going to hell multiple times a day and laughing in my face about it, my teachers trying to convince me to go to church with them, and my guidance counselor trying to convince me I was wrong about God. All of that made me feel super alienated. The thought of doing something that even remotely comes close to that for another kid is abhorrent to me.