The basic problem with teaching creationism in schools isn't that it is completely religious in origin and foundation. It is, of course, but that isn't the real problem. The big problem is that it's wrong. Our schools (naturally) attempt to teach correct facts - in science, they teach correct facts, and possible interpretations of those facts, including the most popularly held. However, creationism is supported by no facts (outside the bible, which is a religious text). If it were supported by evidence sufficient to make it a genuine 'contender' as to origins, it would be taught in schools, regardless of the fact that its origin is religious in nature.
But it's not. At root, that's the biggest problem for the creationists trying to get it taught in schools. It's wrong. When it comes to facts and supporting evidence, it is not a 'contender' as a theory about our origins. It's on the same level as the myths of the Australian aborigines, or the belief that we were sneezed out by a mutant star-goat. It has no support whatsoever apart from religious beliefs. As such, it is not taught.