The 15-year-old boy, who cannot be named for legal reasons, took two rifles to his school, the Atlantis Beach Baptist College in Two Rocks, about 70km north of Perth, on May 24 last year, the Perth Children's Court heard on Thursday.
He fired three shots from his father's .243 hunting rifle, sending students and teachers scrambling for cover as two bullets hit a classroom and another a grassed area near a playground.
No one was injured and the teen was arrested at the scene after calling triple zero and admitting he was responsible for the shooting.
The teenager was arrested soon after the shooting. (HANDOUT/WESTERN AUSTRALIA POLICE)
He later pleaded guilty to eight charges, including two counts of unlawfully acting with intent to harm and endangering the lives of students and teachers.
He also admitted three counts of possessing a firearm or ammunition without a licence, discharging a firearm to cause danger or fear, possessing a prohibited weapon and driving without a licence.
Court President Hylton Quail said it was extremely serious offending that could have ended in tragedy.
He said the boy had caused fear, anxiety and distress for the teachers and students at the school.
Judge Quail sentenced the boy to three years detention, with a term of at least 16 months to be served before he is eligible for release.
Atlantis Beach Baptist College is a co-educational school for boys and girls from kindergarten to year 10.
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