The National Center for Science Education has a report out
on how evolution fares in the states (published in Evolution, Education, and Outreach
Read the report to see how state science standards teach evolution. The authors, in addition to looking at the standards, make an argument for why standards are important:
We therefore return to the primary question: Are standards really that important and/or effective when it comes to improving evolution education? Does it really matter, for example, that some states describe evolution only as “change through time” rather than as descent with modification from a common ancestor? The answer is “Yes.” Even if a good treatment of evolution in state science standards does not guarantee that evolution will be taught well, it provides a critical resource for teachers who want to teach evolution correctly. The clearest example is that a good treatment of evolution in the standards provides important support for biology teachers facing protests from creationist students, parents, and administrators who want creationism taught, or evolution not taught, in life science courses. However, it is also an important support for combating two other problems, experienced by many science teachers who contact NCSE for advice: parents who want their students to be able to “opt out” from evolution-related lessons and creationist teachers of non-science subjects who attack evolution in their own classes. Both of these phenomena have the same educational impact as attacks on or omission of evolution in science class; they leave students ignorant or misinformed about evolution. However, administrators often deal with the latter situations differently, reasoning that if science teachers are not actually being prevented from teaching as they deem appropriate, it is best to keep everybody happy by allowing creationist students to opt out and other teachers to criticize evolution if they want to. A good treatment of evolution in state science standards can help to persuade administrators that the teaching of evolution is not a matter for political negotiations between parents and teachers with different interests but a clear educational necessity. Students simply should not be allowed to opt out of material that the state considers essential (Scott and Branch 2008), and non-science teachers should not be allowed to contradict or undermine this material in their own lessons.
I think they are leaving out one of the most important reasons for putting evolution in the standards. Putting it in the standards is the first step towards putting evolution on the state's standardized tests. If it's on the state tests, it will get taught, even by teachers who don't like doing it. It's important for teachers and schools that their students do well on these state tests, because there are consequences for not doing well. If you test on evolution, teachers will teach evolution.