I think both should be taught.
The creation story is an important aspect of every religion, and every religion should be taught in schools. Learning about other religions helps people to understand other cultures, and understand why some things in history happened. For the sake of religious equality, all religions should be taught in the context of "X people believe that...", because it would be weird teaching them in the context of "God created man and woman. Also Odin, Vili, and Vé crafted man and woman from two trees, and Xenu brought millions of aliens to Earth to kill them with hydrogen bombs and blah blah Scientology".
Evolution is necessary for learning Biology. Regardless of whether or not evolution DID happen, observation has shown that life appears, without flaw, as though evolution happened. Maybe God made the Earth 6000 years ago to appear as though it was much older. Maybe the story of creation in Genesis is symbolic, not literal. We have no way of knowing, and it is not necessary for us to know.
When learning about life, we know that life perfectly has the appearance of coming to exist through evolution, so we must conduct science under the assumption that it did. God would not have made life appear this way without a reason. Maybe it was to make learning about life easier. Maybe it was to help humans discover medicines and such that occur naturally. Maybe it was because God wanted evolution to happen over time, and made life appear as it does to match the appearance of an old Earth.
It's not relevant why God made life the way He did, at least not for our scientific purposes. And it's not relevant if God actually made an old Earth 6000 years ago, or made a new Earth millions of years ago and told the story of creation symbolically. For the purpose of learning about the Earth, we must work under the assumption that the universe is billions of years old, because God made it in a way that it appears to be billions of years old.