Rigby Police Chief Keith Hammon said the boy's parents called police Monday evening when they found the list on an electronic tablet.
"It wasn't a huge list," Hammon said. "All of the parents of the students whose names were on the list have been notified."
Names and how many students and police officers were on the list have not been released publicly. The name of the juvenile taken into custody was also not released. Hammon said police believe no one else was involved in planning the possible attack.
Hammon said officers found a bag of weapons at the boy's home, including multiple knives, handguns and ammunition. Specifics about the weapons, belonging to the parents, weren't released.
Hammon said bullying at the boy's school, Rigby Junior High, was a factor.
"We're pleased to have been able to address this matter before anything came to fruition," Hammon said. "Hopefully, everything will work out for this young man in the long run."
Police planned to transport the boy to the Behavioral Health Center in Idaho Falls for a mental health evaluation.
No time frame was given for the possible attack.
"It was determined, however, that the junior high school was the target of the plot," Hammon said. "Of course, we take any threat seriously."
Students at the school are being encouraged to speak with a counselor if they desire.
"A threat like this tends to put people on edge," Hammon said. "However, our community is still safe."