I think that context is key. If high school students learn about the horrors of slavery in the United States, they also need context like this (the width of the arrow indicates quantity of people transported):
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The overwhelming majority of the African slaves that came to the Americas went to two places: Brazil and the Caribbean (Cube, St. Dominique & Jamaica). The horrifying reason was that the Brazilians and Caribbeans needed fresh supplies because
they kept working their African slaves to death and needed replacements. Once the United States and Britain outlawed and enforced the trans-Atlantic slave trade in 1808,
the number of African slaves in the United States grew from 1.2 million in 1810 to 3.9 million in 1860. That was all natural increase.
Also, a significantly greater number of Africans slaves went to muslim Arab countries than to the colonies that became the United States.
None of that excuses African slavery, but it does add needed context.