I remember when I was in high school and I learned about the Civil War, the basic throughline was that slavery was one of several issues that led to the civil war. States rights, Northern banks, tariffs, and perhaps some other things were the other factors.

I would say the popularity of this theory of the civil war is evident in Simpsons episode 23, "Much Apu About Nothing", which aired on May 5, 1996. Apu takes the American citizenship test, wherein one of the questions is the cause of the Civil War. Apu begins to flesh out a comprehensive answer: "Actually, there were numerous causes. Aside from the obvious schism between Abolitionists and Anti-abolitioinists, economic factors, both domestic and international played a significant--" when suddenly the test administrator interrupts him to tell him "Just say slavery."

The joke is that all the well-educated people know that there were many factors, and no single cause, that gave rise to the civil war, whereas the average "American Idiot" thinks it was "just slavery". Obviously, that joke wouldn't work at that time (1996) if a significant number of people didn't believe the historical reality was complex, and not "just slavery."

But in the past several years savvy interneters have learned that the Secessionists made it clear at the time they were seceding in order to preserve their "peculiar institution" of black chattel slavery.

My question is, when was this adopted as the narrative for American school curriculum? Had it been like that since the Civil War? Or was there an effort to lighten the emphasis on slavery in high school curricula in the past?