I am prtetty sure that reading, writing, math, science and social studies is on the agenda of PUBLIC schools. Leave the religion education to the churches. People can decide for themselves which is right for them; they know where to go.
I agree that instruction in said subjects is the principal role of public schools. However, another aspect of public schools is instruction in ethics and civic responsibilities. Ethics and the duty of a person to others and society at large bring questions of a metaphysical nature into play. Why obey the law or why should I not covet my neighbor's wife or goods are questions that cannot be answered apart from one's beliefs regarding the fate of man and his eternal destiny. There are many Lutherans, Ba'hai adherents, Seventh Day Adventists, agnostics, and Buddhists obey the law and do not covet their neighbor's wife or goods, but for different underlying reasons.
In that regard, having a local pastor or rabbi address students can be considered as part of a good public education. It is true that in the 19th Century, a generic Protestantism was the de facto religion in the public schools in many area, complete with the King James Bible and an anti-Catholic bias in the history textbooks, a fact that generated the rise of the Catholic school system. However, offering the classroom on an occasional basis to a Gideon representative, a Mormon missionary, or an advocate of atheism can be a part of a public education. The key is showing a broad spectrum of views, unlike the case in the era that the public school system was established.