"The students apparently had issues with the teacher," said police lieutenant Kristi Schmitt, of Winder, Georgia. "He sent them to the principal's office and they weren't fond of that."
The unidentified boys warned friends to stay away from Russell middle school on the last day of term, May 21, when the "Columbine-style" attack was apparently planned. They allegedly planned to kill themselves afterwards.
"They said they had a hit list of people consisting of everyone," said Lt Schmitt. Several "crying and hysterical" girls alerted the assistant head after spotting a gun in one of the boys' backpacks at school on May 14, and police arrested them, she added.
Their defence lawyer told a May 25 hearing that the students hatched the plan to impress "some pretty girls" and were just "shooting off their mouths". But the prosecution said they were serious. Police recovered detailed diagrams of the school layout and papers about death and suicide from their homes.
James Mercy, an expert in violence prevention who tracks school shootings for the US government, said 358 pupils were killed between 1992 and 1999.
Killers typically feel picked upon by others, acting "out of a sense of retaliation". "In multi-victim homicides we often find a history of suicidal (thoughts)," he added.