Science deals with conclusions based on evidence observed so far. The conclusions of science are therefore tentative and subject to change. Religion deals with eternal truth and is not subject to change. If creationism is taught in terms of conclusions based on the evidence and not as eternal truth then it is not religion but science and it can be taught in the public schools.
How can this be done? By presenting scientific evidence that is consistent with a creation of life by a supernatural being. The teacher would state that certain scientific evidences are consistent with the theory that a supernatural being created life on earth. However, this is not a religious statement because new evidences could be found that would negate these evidences. Thus the conclusion that these evidences are consistent with a creation of life by a supernatural being, is not a religious statement because it does not assert an eternal truth. The teacher is not saying that God created life, only that some current evidences are consistent with the theory that a supernatural being created life. Also, these evidences do not prove that God created life; they are merely consistent with this assumption, and some of them may pose problems for evolutionary theory. Such teaching would not violate the separation of church and state.
Another way in which teaching creationism is not teaching religion is that creationism does not refer to the beliefs of a particular religion, but refers to the belief that God created life, and this belief is common to many religions.
Furthermore, it is legitimate from a scientific viewpoint to present a scientific theory that states that we do not have any valid scientific explanation for the origin of life, and in fact, as science is currently defined, there may be no valid explanation for the origin of life. The reason for this is that science is defined so as to exclude supernatural explanations, so if in fact life did originate by supernatural means, then there would be no valid scientific explanation for the origin of life. Therefore it is legitimate in a science class, without being religious, to present the possibility that there can be no valid scientific explanation for the origin of life if in fact life originated supernaturally. This can be presented without introducing religion into the science class. Again, this is not a proof that God created life, but simply a presentation of a theory stating that there may be no valid scientific explanation for the origin of life because of the way science is defined.
Perhaps if creationism were presented in this way, the objections to its teaching in the public school system would be overcome. Hopefully this approach will help to resolve current controversies about the teaching of creationism in the public schools.