Roy High School sophomore Bailey Gerhardt told The Salt Lake Tribune she received the text from a friend, one of the suspects, and told one of the administrators, which led to the arrest of the two teens. Roy is about 30 miles north of Salt Lake City.
Gerhardt said Thursday the text from the 16-year-old boy asked: "If I told you to stay home on a certain day, would you?"
That boy, whom The Associated Press isn't naming because he's a minor, and Dallin Morgan, 18, were pulled out of school Wednesday.
"It was the work of a very courageous student who came forward," Roy police spokeswoman Anna Bond said Thursday. "It could have been a disaster."
Gerhardt characterized the 16-year-old as an angry person recently dumped by his girlfriend. She said he had told her he had looked into the 1999 mass shootings at Colorado's Columbine High School.
The juvenile later told investigators he was so "fascinated" by that massacre that he visited the Littleton, Colo., school and interviewed the principal about the shootings that killed 13 people. Roy police said the principal, Frank DeAngelis, confirmed that the boy made his visit Dec. 12.
The Roy High School plot "was months in planning," said Roy Chief of Police Gregory Whinham, and included plans for a device designed to "cause as much harm as possible to students and faculty" at the school, which has about 1,500 students.
The FBI is examining the suspects' computers, police said. Local and federal agents searched the school, two vehicles belonging to the suspects and their homes but found no explosives.
Morgan told police the 16-year-old suspect had previously made a pipe bomb using gun powder and rocket fuel.
"Dallin told me that (the juvenile) bragged about using a bomb to blow up a mail box and having three handguns in his house," a police affidavit states. The 16-year-old boy "claimed that he did not have the guns but Dallin was the source of the guns because he is 18 and can purchase a gun."
The two students prepared by logging hundreds of hours on flight simulator software on their home computers, and they planned to take a plane at Ogden Hinckley Airport after the bombing, Bond said.
Besides hinting at the plan, the juvenile also texted to a friend that both suspects wanted "revenge on the world" and "we have a plan to get away with it too."
He hinted at the plan by writing "explosives, airport, airplane" and added, "We're just gonna kill and fly our way to a country that won't send us back to the US," according to a probable cause statement police filed to make the arrests late Wednesday.
Morgan was being held on $10,000 bail at Weber County jail on suspicion of conspiracy to commit mass destruction. The juvenile was in custody at Weber Valley Detention Center on the same charge. Prosecutors were weighing possible additional charges.
Both students had "absolute knowledge of the security systems and the layout of the school," Bond said. "They knew where the security cameras were. Their original plan was to set off explosives during an assembly. We don't know what date they were planning to do this, but they had been planning it for months."
School officials said there were no imminent plans to hold a school assembly.
The parents of both students "woke up in the middle of a nightmare," Bond said. "They've been very cooperative."