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Marx's influence can be found in a wide range of disciplines because not everything he said was wrong, he was after all a genius, but the economic philosophy of Marxism has been totally discredited and is taught as such.
As to creationism: Darwinism is essentially the theory of evolution, with the alternative being creationism. Teaching Darwin's theory is not religious, it is science, hence it is in there.
One of the things that should be in schools, which unfortunately cannot be there because of constitutional constraints and a definite slippery slope would be teaching the King James Bible in litrature classes.
It is fairly central to a lot of English litrature, but because of the difficulty in teaching the basis of a current major religion (It can't be taught as mythology, because that favours atheism) it is not going to happen any time soon.