Genesis should not be taught alongside evolution because genesis has no basis in the scientific method. It belongs in a religion class at best, certainly not science class. And in fact, unlike genesis (or the flying spaghetti monster), there is proof for evolution. Let me explain (and I borrowed this from myself in another debate):
There is confusion as to the definition of theory, as captured in the statement that "it's just a theory." The scientific definition of theory is a well-supported body of interconnected statements that explains observations and can be used to make testable predictions. It describes a coherent framework that observable data fit into. Conversely, the colloquial definition of theory can mean an opinion, hypothesis, conjecture, or
speculation and does not have to be based on facts or make testable predictions.
A scientific theory is the result of countless iterations of the scientific method which is as follows: ask a question, do background research, construct a hypothesis, test with an experiment, analyze the results and draw a conclusion, and report results or construct a new hypothesis and repeat. Reported results are subject to peer review. This process is repeated by people elsewhere again and again until it potentially reaches the
status of scientific theory.
There is not a bright line dividing fact and theory. Rather, there is a sliding scale of probability. A fact is an idea that is so likely to be true that we do not doubt it and act as if it is true. Evolution is also a fact in the sense that it is so firmly supported by evidence that it is assumed to be true. Fact: fruit flies, which reproduce rapidly, have been observed changing from generation to generation, i.e. evolutionary change, in laboratories. So evolution is a fact in the same way that observations of gravity are facts. Like gravity, evolution is both fact and theory.