Giving evidence at Leeds Crown Court on Wednesday, the mother described the boy, who was dating her daughter at the time of the alleged incident, as "devious" and "primitive".
-> Also in court: Burglars spray liquid in woman's faceThe boy and his younger "friend", are on trial accused of planning a mass shooting at their school in Northallerton, North Yorkshire, after supposedly being inspired by Eric Harris and Dylan Klebold, the teenagers who killed 12 students, a teacher and themselves at Columbine High School, Colorado, 1999.
Addressing the court from behind a curtain, the mother said she saw the older of the two defendants in her daughter's bedroom after she was awoken in the early hours of October 21 last year.
After leaving to get her dressing gown, she said she came back to the room to see the boy running away from the home with a "silver" knife which was "longer than 12in".
-> Don't open this Whatsapp text bomb that could destroy your phone"He was running on the right-hand side of the street. The blade was nearly touching his ankles when he was running up the road," she said.
Describing the image of him with the knife, she added: "It's bad enough, every time you close your eyes you can see it."
The woman denied suggestions by Richard Pratt QC, defending the older boy, that what she saw was a trainer.
She also claimed her daughter had initially "seemed happy" after starting to date the boy in June last year.
However, when he came to stay at their house during the summer holidays, the witness said she "started to have concerns about his character".
She told jurors she and her husband discovered that programmes about "mass killings and shootings" had been recorded on their Sky box, after her daughter had supposedly become influenced by her then-boyfriend's viewing habits.
"We had regularly watched Midsomer Murders and crime thrillers, but nothing like that sort of thing," the girl's mother told court.
Describing the defendant, she added: "He's devious and primitive and had quite a lot of power and control over what, at the time, was our vulnerable daughter."
-> Warning issued as BMW recalls 312,000 models at risk of stalling without warningShe and her husband had "banned" their daughter from seeing him, but on October 14 they discovered a note outside her bedroom which apparently read: "I waited two hours. I love you."
The girl's father said he "came across" letters, supposedly written by the older defendant, which featured drawings of "long-haired zombies ... firing guns" and "holding knives".
The boys, now aged 15, jointly deny conspiracy to murder in relation to the alleged plot, as well as intentionally encouraging or assisting an offence and a charge of encouraging or assisting an offence believing it would be committed.
The older teenager has also pleaded not guilty to aggravated burglary and unlawful wounding.
The trial continues.