I think the major difference is that Darwinism is a theory and creationism is a belief. One is based on evidence and guesses, and the other on religion. Now, it seems obvious to me that religion should not be taught at school. It should not be something you know. It should be something you believe in (or not).
If there was a scientist finding evidence that pointed toward creationism, or a scientific theory formed about it, most probably it would be taught. But there isn't. And you know how man is. Leaps on any ready answer. Until an Einstein comes in and proves it all wrong. :-/