It's a Fact
I think many of us who support the teaching of evolution allow ourselves to get bogged down in semantic games over the meaning of the word "theory." As Patrick Chisholm writes in the Christian Science Monitor,
But saying evolution is a theory is like saying the earth revolving around the sun is a theory. Or that plate tectonics (continental drift) is a theory. Or that things are made of atoms is a theory. Just because such things are not apparent to the human eye doesn't mean they aren't factual.
Evolution is a fact. Because it involves time periods spanning hundreds or thousands of generations, evolutionary change happens much too slowly for humans to perceive.
Chisholm, however, concludes his piece with the old intelligent design canard that organisms have complex parts, and that there would be no evolutionary advantage to having only a portion of one of those parts. In fact, many organisms do have parts that resemble a "rough draft" of a more complex organ.