The then-14-year-old was arrested in 2020 after journal entries boasted of bomb-making knowledge and a plan to "ruin on the school's reputation, traumatise people and get our revenge".

Thomas Eliot, a court-assigned pseudonym, also wrote of thinking "dark things regularly" such as walking into his school with a sawn-off shotgun and killing everyone "whoever f***ed me over".

The Crown alleged he formed an agreement with his best friend to execute his "pure retaliation" plan: spark an evacuation with deodorant can explosions, kill his bully and stab people in the corridors.

But following a five-day trial, a District Court judge this week accepted the severely depressed teen was venting, ranting and did so to soothe and cope with the torment he was feeling.

"Where he writes 'I am a whole lot more homicidal and suicidal' he continues to write 'but I don't want things like that to happen, right. I want to experience the new technology, games and new relationships even career'," the judge said.

"These are not the writings of an agreement to murder and an intention to carry out that agreement."

The Crown's sole witness was Eliot's best mate, who gave evidence in return for immunity from prosecution.

The pair often played tactical shooting video games, sometimes dressing their characters like the Columbine School massacre shooters, the court was told.

The friend said he'd seen a sketch of the school on Eliot's whiteboard that included what looked like dynamite within each building.

Eliot later invited his friend into the massacre plan, spoke about getting head-to-toe black outfits and wrote about being really excited he was getting closer to "PR".

"But it'll be a couple of years I think. I'll create a few pages on PR and list EVERYTHING. It's a conspiracy to murder, careful," Eliot wrote.

However, the friend's evidence about whether he took Eliot's plan seriously wavered dramatically.

Under questioning from Eliot's lawyer, the friend conceded he wasn't seriously agreeing to kill people and would have told an adult if he believed Eliot was going to hurt people.

Eliot's ideations came to light after he spoke to a school counsellor - a meeting initiated by his mate.

Ultimately, the judge wasn't satisfied by the reliability or accuracy of the friend's evidence.

Further, the diary entries had to be looked through the lens of a 14-year-old child, he said.

"The diary entries are the writings of a severely depressed and anxious child who was experiencing a great deal," the judge said.

"The topics are wide-ranging where he wavers at his own thoughts."

Court orders and non-publication laws prevent publication of the children's identities, the school's name and the city they lived in.

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Australian Associated Press