"We will walk into school and at the end of it no one will walk out alive," Ross McKnight, 16, wrote in his diary, according to prosecutors. It would, the teen wrote, be the "greatest massacre ever."
The defense denies the charges but has not yet argued its case.
The jury also was told that a safe in 18-year-old Matthew Swift's bedroom contained plans of the school and instructions on using acetone peroxide as a detonator. The safe also contained a notebook with plans, jottings and an image of the two Columbine attackers taken from closed circuit camera footage taken during the 1999 killings.
Underneath was written: "They say a picture paints a thousand words. This is my favorite picture in the whole world."
Prosecutor Peter Wright said the two British teens "planned to copy and emulate the actions of Eric Harris and Dylan Klebold," who killed 12 students and a teacher at Columbine High School before committing suicide.
"They had discussed, they had fantasized and eventually they had agreed to convert their fantasies into reality," Wright said.
The jury was told that McKnight called a girl in March to tell her that he "couldn't wait until April 20 - the 10th anniversary of the Columbine High School massacre." The girl said McKnight was drunk as he revealed the plans.
The girl, who cannot be named, testified that she believed what he was saying, "just by the way he was saying it." The next day, she informed her mother and they went to police.
McKnight and Swift were arrested. In interviews with police, McKnight refused to comment, while Swift said he was "a confused teenager with a vivid imagination."