The school said sorry to parents after the mother of a black pupil posted the assignment on Facebook.
Our Redeemer Lutheran School in Wauwatosa told parents in a letter that the assignment was offensive and "showed a lack of sensitivity".
Trameka Brown-Berry posted a picture of the assignment after seeing her nine-year-old son struggling to answer the question.
She compared the question to asking someone to list three good reasons for rape or the Holocaust.
The school's letter thanked Brown-Berry for bringing the matter to their attention and said the purpose of the homework was not to have students argue that there are any good reasons for slavery.
