(CNN) When Angela Walker went through her son's school folder, she was shocked to find what the fifth grader had been working on in class.

A social studies assignment from Blades Elementary School in Oakville, Missouri, had asked some students, including her biracial son, to imagine that they work in the slave trade. Then it prompted them to set a price for a slave.

"You own a plantation or farm and therefore need more workers. You begin to get involved in the slave trade industry and have slaves work on your farm. Your product to trade is slaves.

Set your price for a slave. _____________ These could be worth a lot.

You may trade for any items you'd like," one of the questions in the exercise read.

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