The best that I could find was a 2011 Pew Research poll -- Pew being a solid reference. The summary suggests that knowledge of history is a mess. And the Civil War is still important in the high school curriculum.

"Asked their impression of the main cause of the Civil War, a 48%-plurality of Americans say it was mainly about states’ rights. Just 38% say the Civil War was mainly caused by slavery. Another 9% volunteer that it was about both equally. Young people are more likely than older Americans to say that the war’s main cause was states’ rights — 60% of those younger than age 30 express this view, the highest percentage of any age group. Those ages 65 and older, by a 50%-to-34% margin, are the most likely to say that slavery rather than states rights was the main cause of the Civil War."