The business of science is to constantly refine our theories about the world in the light of evidence. The best theory is the one that best accounts for the evidence, rigorously obtained. No scientific theory is the final word on the matter: it is simply the best current explanation for what has been observed. No scientific theory can be proved to be true, but it can be shown to be inadequate - wrong, if you like - by the production of counter-evidence. All science has to be potentially disprovable.
As new scientific understanding emerges, competing theories may also emerge. It is the proper business of science to design experiments to put these theories to the test in order to discover flaws. This in turn leads to the least flawed theory prevailing as the latest provisional explanation.
The reason why creationism cannot be taught as science - as if it were a theory in direct competition to evolution - is that it is not a scientific theory at all. Followers of creationism cannot define what evidence they would accept as disproving their `theory'; doubtless for some even to ponder on this would be to doubt their faith, akin to sacrilege. For them, creationism is simply not disprovable.
On the other hand, there is no reason not to discuss creationism in a science lesson. It provides a very clear illustration of what is and what is not science, and can therefore serve to further the understanding of the nature of scientific progress.
This is not to say that fundamentalist religious belief - such as creationism - is not an important topic of study in its own right. This is particularly the case given the threat that fundamentalism in its various modern world forms poses to enduring shared values such as tolerance, moderation and compassion. But its proper place lies within the teaching of religious education, sociology and psychology, not in science.
Footnote: in the same way, should we need to study the`Church' of Scientology it should be considered within business education, as an example of a successful ethically corrupt money-making scam, not as a religion.