Losing my religion.

[Happy Monkey]

Must have. Every textbook I've seen, as a child, and now in my children's books (if you have kids, you should read their textbooks at least once to see what's in them) present the information as fact. Not one of them said, "This is our best guess."

The state of textbooks is indeed deplorable. See Richard Feynman's autobiographies for his experiences in the approval process. But putting inaccuracies aside:
The "best guess" sentence applies to ALL of science, not just natural selection. A general science book ought to describe the scientific method, and state that all scientific statements can be divided into two piles: data and theories. Data comes from experiments. Theories provide a framework that ties together data from past experiments, in an attempt to predict the results of future experiments. No explanation is fact. there's nothing special about natural selection in this area.

Why not ALSO teach the creation theories of different religious belief systems as well?

Because it's a science class, not a comparative religions class. Creationism was not arrived at by the scientific method. It is not supported by any experimental data. It is, in fact, impervious to experiment. All contrary evidence is ignored as God trying to test people's faith.
It is impossible to rule out a magical being who fakes the evidence, but it is also scientifically meaningless, and therefore worthless as science. If belief in it helps you spiritually, then I have no problems with that. But it is not science.

Evolutionary Theory et al. is a religion.

No more or less than any other scientific theory. Are you advocating the removal of science classes from public schools, based on the fact that science honestly admits that it is not 100% proven? Or are you only worried about science that doesn't agree with current interpretation of your religion?