Not the commenter above, but as a kid I knew about slavery, and about "people owning other people". I even knew about how long it had gone on for, and that slaves were transported in ships around the Mediterranean.

That said, I was wholly unprepared for the brutality of the Atlantic slave trade. Due to the higher round trip times, and lack of mid-trip provisioning, there was a requirement to be more "efficient" than the slave trade across land or sea previously.

At least speaking for myself, my compulsory education only really focused on slavery as an American institution and didn't really address it in a broader context. I obviously picked up a little here and there, particularly since I took Latin all through high school and learned a little bit about Roman culture, but it was only recently I learned how widespread slavery really was.

As an example, it was news to me that slavery was still practiced in parts of the Middle East until the 60s, and probably still exists in a less obvious manner today. I was really horrified to learn that tons of slaves left Africa via the Eastern trade routes, and the brutality of those owners is why there's little to no visible evidence of slavery in those parts of the world. Evidence meaning descendants of said slaves.

Slavery still exists in essence in India and East Asia, with domestic servants working for families as soon as they're old enough to wash dishes.

Everything is new to somebody at some time.