Why should high school curricula include creationism and evolution?
Contrary to popular belief, science does not always back evolution.
Evolution is the belief that life began spontaneously from very basic materials becoming more and more complex forms, until the biodiversity that is seen on the earth today was achieved. There is scientific evidence to support this: the further down in geologic strata one goes, the simpler the fossils found, spontaneous generation of even the simplest life forms has never been observed, the half-lives of naturally occurring radioactive isotopes dictate a minimum age of various rocks (radioactive dating).
However, there are problems with Evolution: spontaneous generation of the simplest life forms has never been observed, when subjected to double-blind studies radioactive dating loses its reliability, there are alternative explanations for the arrangement of fossils in geologic strata.
Creation science uses much of the same evidence as Evolution, but draws different conclusions. For example, the proof that spontaneous generation doesn't occur. Two men, a creationist and an evolutionist are walking along a beach, the evolutionist explaining how life came into existence. Then the evolutionist spots something in the sand, he finds that it is a gold pocket watch. He wonders aloud, "I wonder how this got here." The creationist said, "Isn't it obvious, the components must have eroded out from the ocean floor minerals and come together in just the right order for the pocket watch to form, and then the ocean washed it onto this beach where it became embedded in the sand." The evolutionist said, "that's impossible, somebody had to have made such complex machinery." The creationist replied, "But you say the exact opposite about something as complex as life."
According to creation science, the impossibility of natural spontaneous generation means something unnatural created life; because we can't trust radioactive dating, we can't assume the Earth is necessarily as old as radioactive dating presumes; evidence of Biblical events (e.g. the Great Flood) doesn't get discarded because it agrees with the Bible; The lack of transition fossils (i.e. missing links) suggests that macro-evolution didn't happen.
Since there is evidence supporting and refuting both explanations, and there are even other explanations that fit the scientific evidence gathered, all these explanations must be made known to all. Ultimately, the explanation that best fits the evidence will be chosen by a majority of people, however we don't know for certain which explanation that is.