"This is not a case in which the Ten Commandments are integrated into the school curriculum, where the Bible may constitutionally be used in an appropriate study of history, civilization, ethics, comparative religion, or the like. Abington School District v. Schempp. Posting of religious texts on the wall serves no such educational function. If the posted copies of the Ten Commandments are to have any effect at all, it will be to induce the schoolchildren to read, meditate upon, perhaps to venerate and obey, the Commandments. However desirable this might be as a matter of private devotion, it is not a permissible state objective under the Establishment Clause of the Constitution."