Because Natural Selection is a scientific theory, while ID is theory in the vernacular sense only. Science class is for science. Of course Natural Selection hasn't been proven -- you don't prove scientific theories. Newton's gravitational theory, relativity, thermodynamic theory, none have been proven, yet we live by them every day.
Why do evolutionists find this prospect so threatening?
I'm not threatened at all. Competing scientific theories only add to our understanding of the physical universe. It's no big deal to me if evolution is eventually supplanted by another scientific theory. I think it would be pretty interesting.