Police say that the 17-year-old boy was charged Tuesday in Charleston, S.C., with unlawful communication and third-degree computer crime.
The threat was directed at Concord High School in Brandywine Hundred. Security was increased Oct. 7 at the school after the posting appeared on the website Edmodo.
The posting read: "Don't come to school Monday." It included a photo from the 1999 Columbine school massacre. It also said, "why build for something you're already prepared for, Concord School Massacre 2013."
Delaware State Police investigated the case with Charleston authorities to track the message.
Police say the teen had no connection to Concord High School or to Delaware.