The stabbing took place in April, the last day before the Easter break.
Both men were students at the school.
They met in a locker room after school.
There, the older student tied the 18-year-old's hands, and stabbed him in the stomach.
The now convicted man has confessed to the crime.
According to the district court's ruling, he must pay almost SEK 120,000 in damages to the 18-year-old, Bohusläningen writes.
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A boy was stabbed at a high school in Uddevalla Photo: Robert Betzehag / Rescue Photo