They can't teach Intelligent Design as science because (1) it isn't science and (2) it is based on religious faith, and violates the establishment clause of the Constitution.
The Constitution protects religion, it doesn't prevent it. You cannot have freedom OF religion without guaranteeing freedom FROM religion.
The following is not a rant - it is a statement of facts delivered in a calm voice and fully supported by U.S. legal precedent and demonstrable evidence:
Creationism is not science, regardless of whether it is labeled "Creation Science" or "Intelligent Design".
Evolution is science because:
Evolution is testable:
(e.g. if evolution is true, we should find a fossil for this unknown species in this location)
Example: http://tiktaalik.uchicago.edu/searching4Tik.html
Evolution is falsifiable:
29 examples: http://www.talkorigins.org/faqs/comdesc/section1.html
Evolution has known mechanisms: genetics, DNA, natural selection
http://www.evolution.berkeley.edu/
Evolution is observable:
http://www.nytimes.com/2007/06/26/science/26lab.html?_r=3
http://en.wikipedia.org/wiki/E._coli_long-term_evolution_experiment
Because it is science, because it is the foundation of biology, evolution is taught in our schools (those schools that care about their science education).
Creationism is not science
Creationism offers no mechanisms (how did life "appear" - out of thin air, did the mud rise up and become animals, etc.)
Creationism is not testable.
Creationism is not falsifiable (nothing supernatural can be falsified because by definition it doesn't exist in the natural world, where science works. It's just faith)
Creationism is not observable.
Creationism has been ruled legally to not qualify as science:
http://en.wikipedia.org/wiki/McLean_v._Arkansas
http://en.wikipedia.org/wiki/Kitzmiller_v._Dover_Area_School_District
"Creation Science" and "Intelligent Design" are just labels to make Creationism look scientifically legitimate to people who don't understand science. Without the Bible, there would be no "Creation Science" or "Intelligent Design", so they are driven by faith.
The Intelligent Design movement wishes to change the definition of science so that it will allow supernatural causes. They want to change science. They're not appealing to the scientific community, they're going directly to the public and to politicians using what is called the Wedge Strategy. By publishing lots of books and papers using questionable logic and scientific-sounding language, they're trying to look as legitimate as science to people who are uneducated. According to the Discovery Institute's Wedge Strategy, they target school boards and other politicians to get school science standards changed. In 2007 the school board of Dover, PA, tried to put Intelligent Design into the curriculum. Parents sued and won, and the town was saddled with a million dollar legal bill. The Conservative, religious, Bush-appointed federal judge concluded quite definitively that Intelligent Design was faith, not science, and its introduction to the school curriculum was unconstitutional.
If we allow Christian religious beliefs to be accepted in science education, we cannot keep other faiths out of science and other classes. We can't accept Christian "Creation Science" and exclude Creation according to Scientology. We can't exclude astrology. Only through the application of proper scientific standards can we choose what should be in our school science curriculum.
http://ncseweb.org/
http://en.wikipedia.org/wiki/Wedge_strategy
http://www.expelledexposed.com/