I resisted the temptation to find interviews with the two men so I don't know their personal histories of the song. As such, my answer may miss details from their experiences or different takes on the song itself. Instead, I focused on what was happening in Arkansan and Ontarian education in the mid to late 1950s, early 1960s. Before we get into curriculum specifics, it's helpful to start with the bigger picture around them and some common themes.

It's my understanding that Roberston grew up in Southern Ontario and attended schools in and around Toronto. Meanwhile, Helm grew up in Phillips County in eastern Arkansas. Despite the distance between their schools, they were both surrounded by a culture of anti-blackness and systemic racism. As boys, both men moved through communities that explicitly denied Black children access to the spaces and resources they could access. Although the ruling of Brown v. Board in 1954 was supposed to mean all American states with legally segregated schools were supposed to desegregate, some, including Arkansas, simply ignored the ruling or dragged their feet. In a practical sense to a young Helm, this meant that the adults around him were open advocates of keeping Black children out of white schools. Not long after he'd graduated, white parents founded and funded the first so-called "segregation academy" in Arkansas. Located in Helm's hometown, the Marvell Academy was explicitly created so white parents could avoid sending their children to school with Black children.

Robertson's experiences likely weren't that different. Despite the sense that escaped and free Black Americans would find a different future in Canada, white parents in Ontario were just as willing to stand between Black children and the schoolhouse door as were white American parents. Not only were the student populations in Ontario schools segregated by race, so were the teaching staff such that it made national headlines in 1952 when Toronto hired a Black teacher, Wilson Brooks. Brooks, a former Royal Canadian Air Force Officer, wasn't the first Black teacher in the province, but he was the first Black teacher in front of an all-white class. It wasn't until the 1960s when the provision in The Common School Act of 1850 that allowed for segregated schools was overturned. The primary force behind repealing the provision was a Black MPP named Leonard Braithwaite.

Both men saw segregation firsthand as children. As far as I can tell, neither man had a Black teacher. I'm doubtful that either boy had Black friends or was in community with any Black families. This context matters as it undeniably shaped the curriculum and pedagogy both boys experienced related to the American Civil War and how they responded to that information.

I wasn't able to find specific curriculum documents from Ontario in that era, but it was before the creation of a province-wide history curriculum in the 1970s. That curriculum included mention of the American Civil War but in the context of British North America's involvement, especially as a destination for enslaved people and the role of Canadian men in the Union army. So, it's likely that if Robertson did learn about the American Civil War, the context was focused on the experiences of white Canadian men and possibly, messaging around Canada as a benevolent or welcoming place for enslaved Africans or free Black adults and children looking to escape the war. It is possible that he had a teacher who focused on the experiences of enslaved people before and during the War and as such, centered on the millions of people living in the American South who were very much against the slavery and the Confederation but unlikely. So, it's unsurprising that Roberston was amenable to Helm’s focus on a poor white Southerner.

As evidenced by the song lyrics, when Helms thought of the history of the Civil War, when he sat down to research the history, he centered his thinking and composition on the experiences of white Southerners. Based on his age and where he grew up, I'm confident that not only did Helms bear witness to anti-blackness, but he experienced first-hand one of the most effective PR campaigns in American history: “The Lost Cause” narrative. The idea that the War was about “states’ rights” or that poor, Confederate soldiers were fighting a personal war in way white American men weren’t was advocated by multiple groups, most notably the United Daughters of the Confederacy.

I provide more information about the UDC in this answer here but the gist that's worth stressing is that Helm's teachers - who were surely all white, likely all women - used textbooks that were mandated by white educators, politicians, and advocates who wanted a particular history of the Civil War passed down through the ages. They wanted boys like Helm to empathize with the poor Southern man who lost his farm or had to fight his brother, not the enslaved person whose basic humanity depended on the outcome of the war.

So, to sum up, both men likely learned the American Civil War was about something other than maintaining the system of chattel slavery. They were taught to empathize with the white soldiers, farmers and landowners, to other, or minimize, the enslaved adults and children who worked the land and were compared to property in the founding documents of the Confederacy. They were taught that Lee was someone to be admired, rather than hearing how he was a traitor to his country who enslaved people, including children.