The principal of a Washington, D.C., elementary school says a lesson about the Civil War and Reconstruction era went wrong when some fifth graders asked their black classmates to portray slaves.

The Washington Post reports the Lafayette Elementary School principal detailed the mistake in a letter to parents that explained the learning exercise won’t happen again. Part of the lesson involved students creating a dramatic reading or podcast.

Another letter says students broke into groups and, during that time, several students of color were asked to play “inappropriate and harmful roles.”

The roles included that of an enslaved person and a person of color drinking from a segregated water fountain.