The teenagers, both 15, sat motionless alongside their tearful mothers as the verdicts were read to them today.
The older boy, wearing a shirt, was also convicted of unlawful wounding, but cleared of a count of aggravated burglary.
During the three-week trial, prosecutors claimed that the pair “hero-worshipped” Eric Harris and Dylan Klebold, the teenagers responsible for murdering 13 people at Columbine High School, Colorado, in 1999.
Jurors heard how the boys had prepared a “hit list” of people they wanted to kill, including fellow students and teachers who had supposedly bullied or wronged them.
Analysis of their devices showed that they had researched weapons online and had both downloaded a bomb-making manual.
The older defendant, described as the “leader” of the pair, had supposedly “idolised” Eric Harris, who took up arms with fellow teenager Dylan Klebold and carried out a massacre at Columbine High School, Colorado, killing themselves and 13 others.
The same boy was later found to have kept a diary in which he espoused what prosecutors described as a “far-right wing ideology” and discussed his motivations for wanting to carry out an attack.
The pair were questioned by police officers when, in September 2017, the younger boy told a schoolgirl via Snapchat that they were planning to carry out a shooting.
When she asked if he was joking, he responded: “No. No one innocent will die. We promise.”
The next day, he made what the prosecution described as “clear and unvarnished” confessions, firstly to a teacher, and then to police officers.
During his evidence, the teacher told the court that the boy had said that his targets were “infecting the gene pool” and that he and his friend were performing a “service to society”.
The older boy’s girlfriend claimed that, shortly after that incident, he spoke of a plan to murder her parents and run away together, so that he could become a “natural born killer”.
The schoolgirl, who started dating the boy in June 2017, claimed he described her as “his Dylan Klebold” and encouraged her to give him access to her father’s shotguns.
The teenager, described as “devious” and “primitive” by the girl’s mother, was cleared of one count of aggravated burglary.
He was convicted of unlawful wounding, after carving his name into his then-girlfriend’s lower back.
Officers searched the boy’s “hideout”, where they discovered a rucksack filled with screws, boards, and a flammable liquid which, prosecutors suggested were instruments with which to build an explosive device.
The judge, Mrs Justice Cheema-Grubb, told the pair they will be sentenced at a later date when she has considered reports about them both.
She told the jury: “Nobody will be in any doubt as to the gravity of their conduct and the plans they have made.”
She said it was an “unusual case” but told the jury the “welfare of young people” was the primary concern of the sentencing system.
The pair will be sentenced at Leeds Crown Court at a later date.