It wasn't taught in the sense of "God made the earth in 6 literal days". It was thought in the sense of "this is a theory that is out there and some people believe it".
Teaching it as on par with a scientific theory, and letting students decide for themselves whether they believe it or not, is not anything that is considered scientific, nor should be taught as science. A scientific theory, is not assumptions and guesses. It's a model of the data. It is a scientific model, and as such it is science.
Anything that makes claims about the origins of the earth without using science, is philosophy, or religion. While those may be ways in which to view reality, they are not a scientific reality, nor should be taught in a science class as an option to science. The class is about science, not religion. You don't teach a class that teaches science, and then bring in non-science as science. That is a disservice to the student's education.