A 14-year-old boy accused of plotting a mass killing at his school told a teacher his targets were "infecting the gene pool", a jury has been told.
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Giving evidence at Leeds Crown Court on Thursday, the male teacher claimed that the teenager, the younger of two defendants, had described the alleged plan as a "service to society".
Jurors heard how the same boy had messaged a fellow pupil via Snapchat shortly after the start of the school year in September 2017 to tell her that he and his co-accused planned to "shoot up" their school in Northallerton, North Yorkshire.
The concerned student deleted the messages, but contacted the boy again the next morning to ask about whether the "plan" was a joke, to which he appeared to respond: "No. No one innocent will die. We promise.
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"We're not doing it for a while anyway because we don't have guns atm (at the moment)."
After being shown the messages by the girl, a female teacher passed them on to a male colleague, and the boys were spoken to separately by school staff.
The court heard how how the older of the two defendants, who was also 14 at the time, made "denials about the allegation" that the plan was real.
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However, during his evidence, the male teacher claimed that the younger of the pair said that they had wanted to use bitcoins to buy guns from the dark web in order to carry out the alleged shooting.
The teacher said: "He (the younger defendant) started off by saying that he had sent the messages because some students were making his life in school intolerable.
"He felt that they needed dealing with severely. He said they needed killing.
"He said he needed to eliminate these people, who were infecting the gene pool."
The male teacher went on to say that the same boy appeared to "idolise" Eric Harris and Dylan Klebold, the teenagers who took their own lives after killing 12 students and a teacher at Columbine High School in Colorado in 1999.
"He was emotionless about the plan. He seemed to feel that it was something that needed to be done," the teacher said.
He added: "It was the most dreadful thing that a student has ever said to me."
The same teacher also claimed he was informed by a police officer that his name was one of several on a "hit list" of targets that was supposedly drawn up by the teenagers.
The younger defendant supposedly told him that these people had disrupted him, meaning he was "unable to learn".
The boys, now 15, both jointly deny conspiracy to murder in relation to the alleged plan.
They have each also pleaded not guilty to intentionally encouraging or assisting an offence and a charge of encouraging or assisting an offence believing it would be committed.
The older boy denies aggravated burglary, as well as unlawful wounding.
The trial continues.