It doesn't exist, but it is in the Establishment Clause of the First Amendment to the U.S. Constitution and by legal precedents interpreting that clause. The Establishment Clause states that, "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof."
The "separation of Church and State" is purely a tought of Thomas Jefferson. Peparationists say Jefferson's statement proves that the original intent of the First Amendment was complete separation of church and state, while anti-separationists either consider it irrelevant or might say that it supports the idea that the original intention of the First Amendment was to guarantee religion the freedom to exist without government influence.
It is all interpretation, you interprete your way, I'll interpret it my way. I myself feel that my child should be educated in school on science, math, english, etc. If I wanted a specific religion to be taught, I would send him to a private school. Don't read into this, my son will be going to a Catholic School, not a public school.