The threat prompted officials to lock down all 20 schools in Kershaw County and send officers to several campuses because investigators weren't initially sure which school was being threatened, Sheriff's Capt. David Thomley said.
The 10th-grader was arrested in class at Lugoff-Elgin High School around 10:30 a.m. and admitted making the threats, calling them a joke, according to a police report.
He didn't have any weapons on him, and a search of his home found no guns or bomb-making equipment. But investigators did find notebooks and computer files with disturbing passages referencing the Columbine and Virginia Tech massacres, Thomley said.
"Dark writings - there are mentions of death and just things that typically a rational thinking 15-year-old doesn't write," Thomley said. "You can tell there is some animosity there, some anger - just things you wouldn't expect to see from a 15-year-old who is having a normal day."
The teen was playing a role-playing game on an Internet site early Wednesday morning when he told another player to "have fun watching the news tomorrow" because he was planning to kill everyone at his school, authorities said.
The moderator of the site in England called the FBI, who was able to track the computer to Elgin, about 20 miles northeast of Columbia.
But federal agents couldn't pinpoint the address, and since the threat didn't mention a specific school, officials decided to lock down all the schools in Kershaw County as well as a couple in nearby Richland County, Thomley said.
By 10 a.m., the FBI figured out the house where the threat was made and the teen was arrested, but the lockdown continued as investigators made sure the teen was not part of a bigger plot, authorities said.
The teen confessed to making the threat and he and his mother are cooperating with investigators, Thomley said.
He has been charged with disturbing schools and unlawful use of the telephone - a statue South Carolina has modified to include crimes committed on the Internet. He does not face federal charges and his name was not released because of his age.
The teen, who has Tourette syndrome, remained in custody in a juvenile jail. He hasn't been in trouble with the law before and didn't appear to be having trouble in school, Thomley said.
Thomley thanked the moderator for reporting the threats.
"We don't have any way of knowing where this would have taken us somewhere down the road, if not today, a week or a month later. We just don't know," Thomley said. "Had the moderator not taken the initiative to contact the FBI we might be having a press conference a month from now about something much more serious."