Writing my MA thesis on the evolution of historical narratives in Dutch history textbooks. My supervisors are researching the general censorship of history, and the censorship of history textbooks (or rather: the production and "ownership" of history textbooks). I might be able to give you some interesting points.

I will first answer your question by turning to a different question. You've already put "facts" between quotation marks, which suggests to me that you are already aware of the fickle nature of "historical facts". The statement 'Between 1861 and 1865 northern American states engaged in armed warfare with southern American states' is a statement as close to historical truth as we are going to get. The statement is, also, completely useless. It tells us nothing of the conflict itself. That is where historians enter the fray and, also, bias. A textbook narrative on the 'War of Northern Aggression' could be factually true, but its narrative substance (Frank Ankersmit, Narrative Logic. A Semantic Analysis of the Historian's Language, The Hague 1983) is obviously biased. Narrative substances are not necessarily the plot, but the implicit concept used to narratively group the historical facts within the narrative. E.g. the narrative substance "French Revolution" is what interconnects two or more distinct events, like the the execution of Louis XVI and the arrest of Robespierre.

So much for the philosophy of history and historical fact. As for your question. As /u/wierdjoy mentioned, Texas and California are skewering the production of (history) textbooks. Since Texas and California are the largest markets (with, thus, the largest profits), textbook publishers are thus eager to get certified by the Texas State Board of Education, because only that certification will mean access to the Texas market.

Add to this that textbook revision is an expensive enterprise, most publishers are not interested in publishing America. Our Story. The Not-Texas Edition. So what passes for Texas will, indirectly, pass for the rest of the States. Look at it from the publisher's point of view. It would cost money to make a new edition. And why would they? What's good for Texas ought to be good enough for everyone else. Why cut the profit marges when publishing houses are not in the business of historical accuracy or historical truth?

Economic factors are not the entire problem, however. The accuracy of a textbook is based on a lot of factors. Starting with the limited space available for the actual history in textbooks. I know that US textbooks are often mighty tomes, but in the rest of the world this is far less the case. Through a six year school career, a Dutch pupil might (hope to) read the equivalent of 500-600 pages (excluding "work books"). In 500-600 pages, the usual suspects of history (Prehistory, emergence of city-states in the Levantine, Greeks and Romans, emergence of Christianity and Islam and the Medieval period, pre-modern world, 19th and 20th century) - traditionally written from the national perspective - will have to be included. Dutch history textbooks are (via the government's official Core Aims for Education) skewered towards modern history. So approximately 2500 years will have to be pushed into ~350 to 400 pages of text, of increasing difficulty because first grade pupils are not on the same level in reading comprehension and contextual knowledge as fourth grade students. This leaves about 250 pages for modern national and international (and, a new trend: world) history.

This means you will have multiple narratives running through the same textbook, or even through the same chapter. I have seen this happen in one Dutch textbook where a chapter was, ostensibly, a national account of the events in the Netherlands during the occupation. The authors had made no mention of the events between September 1939 - May 1940, or on the war in general. Yet, when the time in the plot came for the Netherlands to be liberated, the authors suddenly had to explain how the Allies had managed to arrive in the province of Brabant in the first place. Cue an entire paragraph on the war, from Stalingrad to Normandy. The text made a schizophrenic jump from national story to a frenzied political-military international history of the Second World War. One consequence of this was that Japan attacked the USA (because why else would the USA fight the Germans?) without ever mentioning the casus belli. Stranger still, within the textbook narrative, the war in the Pacific was never even concluded!

If the narrative confusion that is the result of different perspectives is not enough, history textbooks also tend to mix and match multiple gapproaches and disciplines of history in their narratives. History textbooks are rarely purely political or social-economic history. They will read as a melting pot of social, gender, political, economic and micro histories. This results in, to put it mildly, ambivalent histories. For more on this, see the Cypriot researcher Eleftherios Klerides. One of the few history textbook researchers to actuall theorize the textbook. See: 'Imagining the Textbook: Textbooks as Discourse and Genre', Journal of Educational Media, Memory, and Society 2:1 (2010) 31-54; and 'Thinking Comparatively about the Textbook: Oscillating Between the National, the International and the Global' in Journal of International Cooperation in Education. While Klerides sometimes falls into the trap of postmodern nonmeaning, he is a great read for anyone interested in the accuracy of textbooks.

So. Are history textbooks accurate? As much as they can be. History textbooks are the result of political, economical and ideological interests. The more interesting question, however, is whether history textbook narratives are legitimate. Perhaps this narrative on the 'War of Northern Aggression' is historically inaccurate, but it has been approved by elected representatives. Democracy approved the narrative. So...

EDIT

One of the first, and by far the best book on the evolution of US history textbooks would be Frances FitzGerald's America Revised. She compares the changing narrative of US history, noting the mirror image of historical narrative: History textbooks tell us as much of the contemporary culture that produces them, than as they say of the past they aim to describe. You could also try James Loewen's Lies My Teacher Told Me, but in my opinion that book is just another contribution to the seemingly endless American History War. An interesting read, but from my point of view, FitzGerald's work is infinitely better.

This is an interesting question. I think there is a way to frame the question that takes it out of the realm of "this might sound like a conspiracy" and into the realm of critical thinking. All textbooks are produced under certain political and ideological frames. Learning to see what those frames are, and especially what kinds of questions are allowable and what kind are considered beyond the pale, is an important part of critical thinking.

One thing to consider is that Wisconsin is too small of a state to exert much control over what gets published in textbooks. California and Texas have a outsized influence on publishers simply by the shear number of students in their public school systems. In recent years Texas especially has been getting a lot of attention for this because the school board has become politicized (source). This was more true in the past than it will be in the future as the publication industry changes and digital textbooks, etc become more common. Simply put, Wisconsin isn't going to be distorting your history textbook nearly as much as California and Texas might be.

For a very interesting read that can help us see the framework of history I highly recommend the book "History Lessons: How Textbooks Around the World Portray US History". The editors looked at the equivalent of a high school history textbook from many different countries. Anytime those textbooks touched on US history they translated that section and compiled it into this book.

What I find very useful about it is that it allows us to read US history through other people's eyes. It does a great job of pointing out important events that just get left out of US textbooks, especially about US involvement in the Caribbean and the Philippines. But it also shows the range of ideological frames. The Canadian and British textbooks were highly responsible and critical, much more so than most US textbooks I would say. While the North Korean textbooks were simply delusional and pure propaganda. The Cuban textbooks were like the North Korean ones with a grain of truth mixed in.