The young man had shown the footage to his classmates, Judge Cook told the court.
The teenager was also overheard talking about school shootings and had a drawing of his school's layout, but Judge Cook determined this behaviour fell outside of the offending and had no sinister overtones.
Police were called to the school after the USB stick and drawing were discovered.
The teenager later told the police the footage was graphic and he wished he hadn't watched it, Judge Cook said.
He downloaded the footage on the evening of March 15 and told police he didn't know it had later been deemed objectionable by the Chief Censor, Judge Cook said.
Judge Cook said the offending had caused extreme stress for both teachers and students at the school.
This morning, she discharged him without conviction, pending further tasks the court is yet to determine.
His lawyer, Amanda Bean, applied for the discharge at his last appearance.
Judge Cook said by discharging him, she was not condoning his actions - nor was it discounting the impacts of the Christchurch shooting and the harm of the footage.
She factored in his age and the level of offending in her decision.
- RNZ