One difference is that the US Constitution doesn’t prohibit establishment of a language, but it does religion.
I think you and I would agree that it is possible to teach an objective class on world religions and include discussions of atheism and agnosticism that didn’t run afoul of the establishment clause. And I also think that we might agree that such a class would be useful.
But I think that the implementation is fraught with problems.
First you have religious persons with exclusionist views who would object to anything but their own version of the truth being taught. Then there would be atheists who would want equal time to cover anti religion (contradictions in holy books, inquisitions, religious wars–arguing that religion is a blight on human history). Finally, just as there are any number of teachers who try to inject religious indoctrination in their classes (contrary to school policy), there will be those who won’t teach the objective class objectively.
I wouldn’t want to be in the situation where every class was videotaped and reviewed by all the partisan groups who had in interest in one religious viewpoint, to insure that no group was treated more equally than another. This why “world religions” is typically taught in colleges where parents aren’t so vocal. And frankly, world religions may be a topic better suited to the maturity level of college students, rather than high schoolers.
Many Americans came here fleeing religious persecution, not the persecution of godless atheists or the ACLU, but the persecution of minority religion by majority religion. The fact that there is an establishment clause, and a free exercise clause in the Constitution indicates that the founders felt that Americans have the right to be religious or not, without government coercion.
Some may be comfortable teaching Genesis alongside “turtles all the way down”, but I suspect they’re in the minority. Would Christians tolerate a world religions program that explained from a psychological point of view why a culture might develop myths about gods for reasons having nothing to do with the objective existence of gods? Exposure to this kind of reasoning has been devastating to the faith of some college students–just think what it would do to high schoolers.