Jury selection was scheduled to begin Tuesday. Bartley, who lost a bid to keep the case in juvenile court, could face life in prison if convicted in the Nov. 8, 2005, shootings at Campbell County Comprehensive High School.
Police say Principal Gary Seale and Assistant Principals Ken Bruce and Jim Pierce were shot after they called Bartley into an office to question him about a student tip that he had brought a pistol to school.
Bruce, 48, died within hours. Seale and Pierce survived, but still carry the bullets inside them.
Defense attorney Mike Hatmaker said the rural community, about 35 miles (56 kilometers) north of Knoxville, is divided in its sympathies for Bartley. The boy, now 15, pleaded not guilty to a seven-count indictment, though Hatmaker has conceded his client pulled the trigger.
Bartley had been in and out of trouble while in middle school and had spent about a year and a half in a residential juvenile treatment program before his parents brought him home.
Deputy Sheriff Darrell Mongar testified during a February hearing that Bartley told him the pistol was his father's and he planned to trade it for OxyContin, a powerful painkiller with a heroin-like high.
Pierce testified at the hearing that he told Bartley he wanted "what you have in your pocket." That was when he said Bartley withdrew the gun.
"Kenny stood up with the gun, waving it at all of us," Pierce said. "Mr. Seale asked him if it was real. He said, 'Yes, it's real. I'll show you. I never liked you anyway."'
Pierce said Bartley pulled out an ammunition clip, loaded the gun and started firing. Seale was shot first, in the lower abdomen. Bruce was shot in the chest. Pierce was hit in the chest as he struggled to disarm the youth, he testified.
The trial was expected to last four days.
World's most powerful nuclear submarines, Arkhangelsk and Severstal, are to be dismantled after 2020 - their further exploitation is unprofitable
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