Another aspect of this "hidden curriculum" that we should all be aware of is that textbook publishers often tailor their content to the states with the largest populations, since those are their biggest markets. This means that if and when a state like Texas seeks to impose its political agenda in education, the textbook publishers are often happy to go along with the program. Since textbook publishers do not publish separate textbooks for different states, because if they did they would lose the economies of scale, in publishing one or two high school history books, for example, this means that the rest of the states do not have much choice in the matter. Thus, not only is one state's students getting content that is often biased and sometimes even just plain wrong, the rest of the states have little choice but to adopt that particular textbook.
This difficulty arises not only in history books, but also in science books, for instance when creationism or intelligent designed is presented as a legitimate and scientific theory of the origin of man. These biases might be overt or quite subtle, as I took note when my younger son spent a (wasted) year in a cyber-school. Anyone looking at his world history text would think there were no cultures worthy of note but the Greek and Roman, and his literature readings were strikingly lacking in diversity. Had it not been for parents who knew better, who knows what he would have made of the world around him?
In the world of education today, it is difficult to know how a teacher can or should proceed to correct for these skewed versions of reality. Since I teach at the college level and have complete academic freedom, it is easy for me to point out where a textbook is not presenting the whole picture or even to choose another textbook for my students. But if I were teaching in K-12, in these situations, I like to hope that I would be teaching subversively as best I could.