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Students at Dover High School in New Hampshire were recorded in class singing a KKK-themed jingle with lyrics that included, "KKK, KKK, let's kill all the blacks," sparking a district investigation.

Dover School District Superintendent William Harbron told NBC News on Monday that the students were 11th-graders assigned to create a jingle based on the post-Civil War Reconstruction era for a U.S. history class. The Ku Klux Klan, a white supremacist organization, was formed in this time period.

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Video began to circulate of the presentation Friday in which students sang, "White masks on our heads, blood beneath our feet, laughing till they’re dead -- ha, ha, ha," to the tune of "Jingle Bells." The students did not know they were being recorded, the superintendent said.

"The principal became aware of the situation late Friday and began working with teacher," Harbron told NBC News on Monday. "Right now we’re dealing the immediate need of communication out," and with "the emotions surrounding the situation."

The district released a letter on Monday stating that it was investigating the incident and that although the song was part of an assignment, "the impact was harmful."

"We are deeply concerned that an event such as this could occur and understand the emotion and concern that this event will create for our students, families and staff," the letter read. "Administration from Dover High School and the District are working with students and the school community to respond immediately and effectively to this racial insensitivity."

Harbron was not aware of any previous disciplinary issues involving the students and said the district wanted to learn from the incident.

He told NBC News the district was working with two consultants from a foundation focused on public education to address the class assignment. The foundation began working with school district earlier this year as a part of a grant to develop community engagement.

"I think the most important part is that we are upset and concerned about what happened in that classroom," Harbron said.