The Score on Religion in Schools [Last Modified on 2006.12.03]
Because schools are government funded and government run, they must also abide by the separation of church and state as suggested by the Establishment Clause.
Creationism — or its allegedly scientific cousin, "Intelligent Design" — is religion, clear and simple. There are no scientific or secular claims to support such beliefs. Therefore, they cannot be endorsed by our secular government in our public schools. Just as a court of law would not accept the notion that "God made me do it!" as a legal defense, our science classes cannot accept the notion that "God did it!" as a scientific theory.
The bible can certainly be taught as what it is: literature. Any attempt on the part of a teacher or faculty member to affirm the "truth" of the bible is an endorsement of religion, and since they are acting as agents of the government, this violates the Establishment Clause. There are very clear guidelines to identify and avoid such violations, which have been agreed upon by numerous secular and religious organizations.
Asking for "equal time" is bullshit, since the fundamentalist Christians have no interest in granting equal time to other beliefs (Taoism, Shintoism, Hinduism, Buddhism, Paganism, Wicca, Greek Mythology, Norse Mythology — just to name a few). When the fundamentalists can figure out why they don't want equal time for these other beliefs, they might learn why we will not give equal time for their beliefs.
It is perfectly acceptable — and even laudable — for those who wish to have their children learn religious beliefs in school to either send their children to private, non-government funded schools, or home-school them. The secular education system is not — despite what the fundamentalists would have you think — "pushing a belief system" on its students; it is simply instructing them in the scientific method, and the knowledge that humanity has gained by using it.