Why is evolution taught as a fact in school?

It is a fact.

When much of the public uses the term "theory" they often mean something that is unproven or an educated guess. We say, "It's just a theory." When scientists use the term "theory" they mean something quite different, and this is what has confused the public about the theory of evolution. Science also has a theory of gravity. That doesn't mean its a guess, it means that the concept of gravity organizes and makes sense of the observations in the real world: release a rock and it falls to the earth.The theory of evolution is the best explanation of the existing data that has emerged from multiple fields: biology and all of its various sub-branches, genetic research, palaeontology, and so forth.

Evolution is not a hypothesis, it is a theory, like the theory of gravity. No theory is ever finished, because new data may emerge to force alteration. Theory is the highest order of scientific organization of data. It allows for multiple hypotheses that can then be tested, and allows for the data to be integrated into a larger picture. The vast majority of life scientists endorse thetheoryof evolution because it makes the most sense of the data. (Many also retain their religious beliefs.) Many of the scholars who challenge evolution have advanced degrees in other fields of inquiry, not in the biological fields that directly deal with the data of evolution.