"So how can it be taught as fact in the first place?" I believe you are referring to the biochemistry and the mechanisms of evolution, not that biological evolution is a fact. That can easily be taught... One example:
"Critics of evolution relish in emphasizing the complexity and unsolved problems surrounding its mechanisms. They point to errors made by earlier researchers, and enthusiastically conclude from all of these difficulties that evolution is “only a theory.” They routinely misinterpret or ignore the repeatable, observable, measurable scientific evidence that overwhelmingly supports evolution as a fact. This evidence is abundant. It is the worldwide, sum total of the fossil record. Sedimentary rocks and the fossils preserved in these rocks, even though an incomplete paleontological record, are primary, factual evidence of what has happened back through time. One thing apart from all others presents a compelling case for evolution that is independent of its weaknesses. It is the simple, repeatable observation that deeper, older rocks contain fossils of more primitive, less-evolved organisms than do the younger rocks that rest above them. Consider the magnificent Grand Canyon in Arizona. The rock layers at the base of the gorge are undeniably older than those at the top. No matter how much time was involved in cutting the gorge through all these rocks, there is no way of reasonably denying that as one takes the winding path down the gorge and passes across the layers of rock one also goes back through recorded time. Today we can observe the same thing, obviously on a vastly smaller scale, in the layers of trash at waste disposal sites. The old newspapers, bottles and cans, the less “evolved” cameras and telephones, 78-rpm records, and vacuum-tube radios are preserved among the layers found toward the bottom. The younger more evolved “fossils” are among those found near the top. In the vast Grand Canyon geological “dump” the deeper flat-lying rock layers contain the simpler, more primitive fossils. Gaps in the record notwithstanding, there are shells buried there that record the presence of species not found in the younger rocks above, and no bones of any kind are to be found. The rocks at the top contain fossil remains of still more evolved (developed) animals. All of these thousands of feet of flat-lying rocks rest directly on still older layers. Those below are set at a steep angle. Obviously, substantial time must be allowed for these even older sediments to have been deposited, hardened, turned on edge by mountain-building processes, and later cut down by erosion. In these much older rocks there are precious few fossils of any kind. There are neither shells nor bones. Only a few fossils of simple, microscopic life forms have been found. Geologists find similar rock sequences in many regions of the globe. While these may not have the grandeur and ease-of-visibility that the deeply cut Grand Canyon offers, their included fossils exhibit the same upward changes. I find it hard to escape the conclusion that from this overall geological and paleontological evidence there has been an increase upwards through time in the complexity and sophistication of life. The global fossil record does not present a jumbled mixture of large and small, older and younger, as would be the case after a giant flood of some kind. This record is repeatable, undeniable factual global evidence for a long process of evolution…of biological change over time. The total fossil record reveals these same upward changes, irrespective of the mechanisms that caused them."