Some parents reacted angrily when they attended the school and saw the posters hanging in a hallway.
The principal apologised for any pain or offence caused.
District officials said children needed to learn about the “uglier parts of our past”, but accepted the posters should not have been hung without context.
The assignment had been set by the South Mountain Elementary School in South Orange, near Newark.
One of the posters depicted “available slaves”, with a drawing of a 12-year-girl called Anne, offered as a “fine housegirl”.
One angry parent wrote on Facebook: “It is completely lost on me how this project could be an effective way to teach any student in any age group about American history.”
School principal Alyna Jacobs apologised for “any unintended pain, anger or offence caused by the assignment”.
District Superintendent John Ramos said the project had been going for 10 years. It is unclear if it will now stop.
Mr Ramos said “anti-bias experts” had been consulted and they had “highlighted the fact that schools all over our country often skip over the more painful aspects of American history, and that we need to do a better job of acknowledging the uglier parts of our past, so that children learn the full story”.
But he admitted: “We completely understand how disturbing these images are, and why parents were upset. This was exacerbated by the fact that the displays did not include an explanation of the assignment or its learning objectives.”
The posters have been removed from the hallways.