New Jersey high school students dressed as an African American slave and owner at a weekend Halloween party, prompting the school district to plan diversity-education sessions, officials said.

The students, who attend Watchung Hills Regional High School in Warren, Somerset County, dressed in the costumes - one student in blackface and the other holding a whip - at an event held off campus, according to a letter from district Superintendent Dr. Elizabeth Jewett.

“Although this incident occurred outside of school and the event was not affiliated with WHRHS, we are undertaking a thorough investigation into this matter, working closely with law enforcement,” Jewett said.

The district said it condemns the students’ conduct “in the strongest terms possible” and planned to address it during the school day. Officials from the district are investigating and working with law enforcement.

Jewett said the school would hold optional diversity sessions Wednesday during students’ free periods as the first step in addressing the incident.

“The Watchung Hills Regional High School District seeks to develop ALL of our students into young adults who are able to demonstrate empathy, cultural sensitivity, and ethical decision-making,” she wrote. “Toward that end, rather than responding to a hateful act with more hate, I am asking that we, as a community, work together to ensure that the racial insensitivity and inappropriateness of these students’ actions serves as a teachable moment.”

The local Echoes-Sentinel said a photo of the teens in costume was posted in a local Facebook group and later removed.

Joe Brandt can be reached at jbrandt@njadvancemedia.com. Follow him on Twitter @JBrandt_NJ. Find NJ.com on Facebook.

Have a tip? Tell us. nj.com/tips