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Darwin's theory was based on evidence and is largely accepted in the biological sciences as one of the pillars of modern biology. It is taught because it is science.
It is accepted by the Catholic Church on the same basis.
Creationism, is not science. It doesn't even really qualify as a scientific hypothesis, as it is not testable. Darwin's theory of evolution is (EG: treatment resistant diseases are a direct result of evolution) and is fairly central to our understanding of life on earth right now.
You might as well object to Geography teachers teaching children that the world isn't flat.
As to Marxism, that more or less points out how damned stupid you really are. What is taught about Marx is that ultimately his theory didn't work, the communist states are painted as failures because for the most part, that is precisely what they were.