Evolution should not be taught in high school science classes because it is not a scientific theory.
The modern scientific method is defined in terms of hypotheses, theories and laws. The difference between each is the level of acceptance in the scientific community. What they all have in common is that they must be falsifiable. This means that it must be possible to run an experiment that would prove the theory (or hypothesis or law) wrong, if it were not true.
Empiricism (a basis in experiment) is what gives science it's credibility. It means that a scientist in Poland does not have to take your word for it - they can do their own experiment and attempt to disprove it for themselves. The falsifiability part prevents people from coming up with theories that can only be proved right. Evolution fails both of these tests. There is no experiment that can test the theory. Any new evidence that comes to light cannot disprove the theory - only either back it up or call for a modification of the evolutionary tree or a modification of the theory.
Natural selection is a scientific theory. Evolution differs from natural selection by including the ideas of common ancestry and beneficial mutation. Just because a theory is not scientific does not mean that it has no merit. However, claiming that a theory is scientific lends it undeserved authority and diminishes the authority of science.
The modern scientific method arose during the scientific revolution - after the renaissance. Observation of nature and speculation do form part of the scientific method. That is how new hypotheses are formed. However they should be immediately checked to see whether they are scientific or not.