24-year-old Reece Elliott sparked a huge US security scare when he posted the abuse in the months after a high school massacre in America.
It resulted in an FBI manhunt, armed guards in Tennessee's schools and thousands of children being kept off school the next day.
He left the threats under a false name on a tribute site for a 17-year-old Tennessee girl who was killed in a car accident.
Elliott, a father-of-one, has admitted one count of making a threat to kill and eight of sending grossly offensive messages.
He has been held in custody since his arrest in February.
Reece Elliott, of Fossway, South Shields, is due to be sentenced at Newcastle Crown Court.