The threat prompted the district to close the school, in the Holland section of the township, affecting 2,200 students plus faculty and staff. It was specific to that campus, Superintendent Mark Klein said, so the 9,360 students in the district's other schools were not affected.
Four other high schools in lower and central Bucks County also were targeted Thursday with threats that were investigated and discounted, and classes continued as scheduled, officials said.
In the Council Rock case, the ninth grader was being held in the county Youth Detention Center in Edison on charges of making terroristic threats and possession of a firearm by a minor.
Police did not know why the threats were made, Pilla said.
The student's mother, Lizabeth Donohoe, 50, was being held at the county prison on charges of possession of a firearm and endangering the welfare of children.
Donohoe said she had given her grandfather's semiautomatic handguns to her son, according to police court records.
The ninth grader made statements Thursday at the school "indicating that he was going to bring a gun and knives into the school building Friday morning and randomly kill students and staff," Pilla said.
Another student alerted his parent about the threats, and the parent called police, Pilla said.
"He's a hero," Klein said about that student.
The ninth grader did not post threats on the Internet, but other students wrote about them on Facebook, Pilla said.
At the ninth grader's house on Cypress Avenue, police found functional 9mm Browning and FB Radom handguns and the replica AK-47 rifle in his bedroom, Pilla said. There were empty clips for each handgun, but no ammunition, he said.
The replica rifle was so authentic that if the student "confronted law-enforcement officers with that weapon, they would shoot him," Pilla said.
Police also found other replica guns that fire pellets, plus knives, machetes and swords throughout the house, he said.
Another search Friday turned up more knives and replica guns, "but nothing of substance," Pilla said.
While police were arresting the student and his mother, the teenager's father showed up and was arrested on charges of resisting arrest and obstruction of justice. Vincent Mario Russo Jr., 48, of West Street Road in Feasterville, was released and will receive a summons, Pilla said.
The mother also was charged with possession of marijuana and drug paraphernalia. She was being held on $100,000 bail.
Pilla and Klein would not say whether the student had been in trouble before Thursday. His name was withheld because he is a minor.
Police dogs were used to search the school, and no evidence was found, Pilla said.
The school was cleared Friday morning, but the district had notified all families at midnight that South would be closed as a precaution.
Authorities also received threats Thursday about Neshaminy, Pennsbury, Bensalem and Central Bucks South High Schools, county Emergency Services Coordinator John Dougherty said Friday.
At Bensalem, "police assured us the threats were unfounded and it was safe to bring students in," said Susan Harder, administrative assistant to the superintendent. "There is extra police on campus. . . . We continue to be on alert."
Police departments and school districts around the state have been tracking threats since last week's attacks in Connecticut, Dougherty said.
Most of the threats have been hoaxes, but they're all taken seriously "because you can't take a chance, especially with schoolchildren," he said.
At Spring-Ford High School in Chester County, a student upset about a grade threatened Wednesday to "make last week look like nothing," Limerick Police Chief William Albany said.
The student had no weapons at school or home, so police and school officials did not consider the situation dangerous, he said.
The student was charged with making terroristic threats, harassment, and disorderly conduct. He also faces disciplinary measures from the school, according to a district statement.
In Felton, Del., a 16-year-old was arrested Thursday for taking a BB gun to Lake Forest High School, state police said.
The student, whose name was withheld because he is a minor, was charged with carrying a concealed dangerous instrument, a misdemeanor, and was released to his parent, police said.
Contact Bill Reed at 215-801-2964, firstname.lastname@example.org, or follow on Twitter @breedbucks. Read his blog, "BucksInq," at www.philly.com/bucksinq.
Inquirer staff writers Mari A. Schaefer and Carolyn Davis contributed to this article.