Reece Elliot, a 24-year-old father-of-one from South Shields in northeastern England, posted the messages on a tribute site for a Tennessee teenager who was killed in a car accident.
"My father has three guns. I'm planning on killing him first and putting him in a dumpster," he wrote on the RIP Caitlin Talley social networking page, using a false name.
"Then I'm taking the motor and I'm going in fast. I'm gonna kill hopefully at least 200 before I kill myself. So you want to tell the deputy, I'm on my way."
Elliot was arrested in February after handing himself in to police in Britain following investigations by the FBI and US Department of Homeland Security, Newcastle Crown Court heard.
In the atmosphere of heightened security following the Newtown school massacre, Elliot's threats had sparked a lockdown of local schools, prosecuting lawyer Gary Buckley told an earlier hearing.
"I am told that the local authority immediately put all the local schools in the area on what was called lockdown. Because of the hysteria caused, around 3,000 children were kept off school on that specific day," he told the court.
He said that when Elliot handed himself in, "he said he was a part-time (Internet) troll. He said he decided to post offensive comments to see what kind of reaction he could provoke".
Buckley continued: "He confirmed he did post the postings on Facebook and therefore did make these threats, but he didn't expect the threats to be taken seriously and didn't expect them to cause the reaction they did."
Elliot's threats were included among highly offensive messages he posted about 17-year-old Talley, a popular pupil at Warren County High who had died in a car accident in October.
He then posed threats about driving into school, writing: "I'm killing 200 people minimum at school. I will be on CNN."
On Friday, Elliott admitted one count of making a threat to kill and eight of sending grossly offensive messages.
Judge James Goss delayed sentencing until June, but warned the defendant that a prison term was "inevitable".
