Harmer-Gemmell was one of four others aged 16 or under who were charged in the Youth Court, from which he was sent to the District Court to be sentenced as an adult offender.
Donner was bombarded with stones, blocks of concrete, bottles and wood, and punched and stomped while on the ground during the assault, which developed after the group of teens on a night out stealing came across their eventual victim who was looking for cigarette butts around the bar complex, otherwise known as the Flaxmere Tavern.
Crown prosecutor Steve Manning noted both the assault and the previous offence, which happened on February 17 and included the theft of a phone, involved stomping and kicks to the head.
Judge Mackintosh said there was no suggestion Harmer-Gemmell took part in the murder but he attacked Donner when he was vulnerable and on the ground.
She declined defence counsel Kirsten Monk's application for permanent suppression of Harmer-Gemmell's name.