All posturing and politicizing aside...
Is it fundamentally moral to compel men to pay for the advancement of religious ideas they do not approve of or share?
Yes. Because we are a diverse society, you cannot exclude one idea while allowing the promotion of the others. That is a back door suppression.
For example: We let Christians have a prayer service in the auditorium but deny that privilege to Muslims.
Amendment V -- No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a grand jury, except in cases arising in the land or naval forces, or in the militia, when in actual service in time of war or public danger; nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.
80% of my church's congregation home-schools. (It's the Orthodox Presbyterian way!!)
They receive zero compensation whatsoever for all the taxes they pay in support of public schools.
Ergo, the public school edifice is Unconstitutional; so naturally, the same goes for any sloganeering (religious or otherwise) plastered thereupon.
I've no objection whatsoever to putting "God Bless America" banners all over the sides of a school.
It's the Public School itself, to which I object.
That's a very good question. But I'll raise you one...
Is it fundamentally moral to compel men to pay for the advancement of ANY ideas they do not approve of or share?