The boys, ranging in age from 16 to 18, were arrested Thursday, the, just hours before they planned to shoot fellow students and school employees, authorities said.
"What the resounding theme is: They were actually going to do this," Cherokee County Sheriff Steve Norman said.
Even as the numbers of such Columbine-style shootings increase, many parents are still not ready to believe that it could happen in their own school districts.
"The only thing that scares me more than a kid with a gun is an adult who believes that it can't happen here," Ken Trump, a school security consultant, told CBS News correspondent Cynthia Bowers.
The four who were younger than 18 were being held Thursday at a juvenile detention center in Girard. The 18-year-old was in the Cherokee County Jail. No decision has been made on whether to charge the four juveniles as adults, Kline said.
The teens planned to wear black trench coats and disable the school's camera system before starting the attack between noon and 1 p.m. Thursday, Norman said. Sheriff's deputies found guns, ammunition, knives and coded messages in the bedroom of one suspect and documents about firearms and references to Armageddon in two suspects' school lockers.
Apparently, they had been plotting since the beginning of the school year. Norman said school officials began investigating Tuesday after learning a threatening message had been posted on MySpace.com.
"The message, it was brief, but it stated that there was going to be a shooting at the Riverton school and that people should wear bulletproof vests and flak jackets," Norman said.
The message also discussed the significance of April 20 as Adolf Hitler's birthday and the anniversary of the 1999 Columbine High School attack in Colorado, in which two students wearing trench coats killed 13 people before committing suicide.
School officials identified the student who posted the message and talked to several of his friends, he said.