Prosecutors claim that the boys, who were 14 at the time of the alleged offences, had planned a mass shooting at their school in Northallerton, North Yorkshire, in a supposed bid to emulate Eric Harris and Dylan Klebold, the teenagers who killed 12 students, a teacher and themselves at Columbine High School in Colorado.
Giving evidence at Leeds Crown Court on Tuesday via a pre-recorded interview, a schoolgirl claimed that her friend, who started dating the older of the pair in June 2017, once told classmates: "Guess who's dating the school shooter."
She also claimed that her friend had said her then-boyfriend was plotting to use "his dad's rifle" to carry out a shooting at his school.
"It was obvious that they (the older boy and his former girlfriend) had talked about it, because she said to me that he was going to shoot up the school," the girl said.
"She quickly said that she was joking, but she talked about all the stuff he said he had under his bed, like chlorine bombs and lists of people he wanted to kill."
Boys plotted to kill pupils and teachers in Columbine-style massacre at Yorkshire school, court toldDiscussing whether she believed the alleged plot was more than just a fantasy, she said: "I did not want to, but I thought that she could not be making it up for no reason."
When asked why she did not report the matter to police, the girl added: "From what I could gather, (the older boy) was insane and I did not want anyone I know or myself getting hurt by him by reporting him."
She claimed that the same teenager had his Instagram account taken down after posting up images of "decapitated children", and once had a tearful "breakdown" in front of his former girlfriend whilst standing on a wobbling board at York dungeons.
The girl also said that her friend had told her that the boy had carved his name into her lower back, and said that she was shown the marks that supposedly followed.
Describing this, she said: "I was trying not to cry.
"I did not ever think that she would get her crazy boyfriend's name carved into her back."
During her evidence, the girl told court how the younger of the boys was an "outcast" but seemed "nice".
The boys, now 15, both jointly deny conspiracy to murder in relation to the alleged plan to kill pupils and teachers at their school.
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 unlawfully wounding his former girlfriend.