At what grade level would you deem it appropriate to expose students to the viewpoints and perspectives of mid-ninteenth century southern United States as it regarded the institution of slavery?
Or for that matter at what grade level should students at a private Christian school begin to read and understand the Epistle to the Collosians?
That's not what this booklet does. I have no problem with students reading contemporaneous accounts via slave narratives, viewpoints of slaveholders, etc. and discussing them in class.
This booklet "Southern Slavery, As it Was" (the very title indicates it's selling itself as the "truth" about how slavery was), "a booklet that attempts to provide a biblical justification for slavery and asserts that slaves weren't treated as badly as people think", is not a contemporaneous account from one side or the other. It's a recently written pamphlet with a particular viewpoint about history and it asserts things like:
"Slavery as it existed in the South was not an adversarial relationship with pervasive racial animosity. Because of its dominantly patriarchal character, it was a relationship based upon mutual affection and confidence." (page 24)
"Slave life was to them a life of plenty, of simple pleasures, of food, clothes, and good medical care." (page 25)
That is not akin to reading Mein Kampf as an insight into Hitler's thinking. It is more akin to reading a pamphlet written by a recent holocaust denier.
I think it has no place in the classroom and I would pull my child out of any school that used such a pamphlet.