I agree that the subject should be taught, but not at an early age. I'm not even sure high school would be the time for something so controversial. As a college level course, such a study of the foundations of religion, not any particular religion, should be, and I believe generally is, an elective course.
The problem with trying to teach these things at an earlier age is that in many cases the parents and churches are busy teaching (read brainwashing) their kids into their own particular brand of religion. Any kind of study which shows how religions develop and evolve would generally, I believe, raise serious questions which the parents, and especially the churches, don't want the kids to ask. You then wind up with the parents and churches fighting with the educators and the kids wind up suffering through a general lack of education. Not too different from what is happening in this country (USA) now.