Evolution is not the TRUTH. Nor has anyone ever claimed it to be. It is a theory, hence the reason it is called the THEORY of Evolution. Evolution is an important scientific theory, if for nothing else, because a lot of other science is based off of it. It also happens to be a pretty good explanation to describe a set of given observations.
A scientific theory isn't taught because of it being right or wrong. It is taught because is the best knowledge we currently have. Science is not a static truth, it is an evolving body of knowledge, that can only evolve if it is actually taught, and most importantly can only evolve because it is TESTABLE. I can design, construct, and than conduct scientific experiments which examine the theory of evolution.
Intelligent Design is not a theory. It isn't meant as something that might be true. It isn't meant to be tested. It is simply meant to be the TRUTH. There are no scientific experiments to be conducted on it. There would be nothing wrong with teaching alternative scientific theories that explain the same observational realities explained by evolution, but Intelligent Design is not an alternative scientific theory, it is an alternative theological truth.
Theological truths belong in theology classes which do not have any place in public schools.
Even if someone were to convince themselves that creationism (a theological truth) belongs in science class, don't they see the logical result of that would be to including other theological truths such as that maybe the universe was created by the exhalation of breath from the god Vishnu? I think most Christians would object to such a thing which means they aren't truly interested in science, as they purport, they are interested in spreading their religious ideas. Missionarianism has no place in our classrooms.