The study of evolutionary processes should be taught in science classrooms, because current evolutionary processes can be studied with the use of the scientific method. Evolution as a theory for the origin of the earth, should not be taught as 'science' because it is really historic speculation. Science can only affirm how 'evolution' occurs now, under the variables that we live under. Scientists cannot use the scientific method to affirm evolutionary processes over hundreds of thousands of years. The most you can get from science is "Based on what we observe about evolutionary processes that occur today within this incomparibly minute time frame compared to the overall 'evolution' of the earth, we can speculate that maybe, possibly, evolutionary processes occured in this same fashion many many years ago." That is speculation, it can never be proven, and it can never be verified by the scientific method.
Both evolution and creationism are speculation. Evolution uses scientific observations to speculate an elaborate theory, Creationism is based in written documents. Both require a great amount of faith, and the debate between the two belongs in a philosophy classroom, not a science classroom. IMO, neither arguement is more credible than the other.