It's religious discrimination.
Teaching creationism privileges a single religious viewpoint. Most mainstream Christians, Jews and Muslims, along with Hindus, Buddhists, deists, and those of other faiths, reject many or all of the doctrines held by self-styled creationists.
Covering the entire spectrum of religious beliefs about origins might be appropriate for a comparative religion class, but it is not appropriate for science classes.
Furthermore, seemingly neutral policies such as "teach the controversy" or "teach both sides" actually single out particular religious viewpoints and place them in opposition to mainstream science. Students could come away from their science classes with the understanding that, for instance, fundamentalist Christianity is particularly hostile to modern biology. Thus, policies requiring or allowing the teaching of creationism could lead to discrimination against the very religions they are intended to defend.
The First Amendment to the U.S. Constitution forbids the government from establishing religion. This mean that public schools can not give special privileges to any religion. Since creationism is a sectarian religious view, it cannot be given preferential treatment by any government body, including public schools.
It’s bad science.
Standard creationist claims — about the age of the Earth, the pattern of descent of living creatures, and human history— are contradicted by scientific evidence, and were rejected by mainstream science over a century ago. Teaching such claims in science class misinforms and mis-educates students.
It would damage our students' understanding of evolution, one of the most significant theories in science, critical to unifying biology, to integrating biology with geology and astronomy, and to establishing the scientific foundations of modern medicine and agriculture. Creationist arguments also frequently promote severe misunderstandings in other scientific and mathematical areas, such as thermodynamics and statistics.
Creationist claims also create confusion about the nature of the scientific research, the way scientists actually obtain and evaluate evidence, make and test hypotheses, and continue to deepen our scientific understanding of natural processes. Thus, the damage of teaching creationism goes deeper that the specific subject matter, and makes it harder for students to understand new scientific ideas throughout their lives.