The Crown has appealed against a 2 1/2-year prison sentence given to a Taradale High School student who abused another student with a broom handle.

Deputy Solicitor-General Nicola Crutchley yesterday questioned in the Court of Appeal whether the sentence imposed on Andrew James Castles in the High Court at Napier on March 19 would have been higher if the victim had been a 17-year-old woman instead of a man.

In February, Castles pleaded guilty to one charge of sexual violation and one of the attempted sexual violation of the student at an unsupervised party last October 17.

His six co-accused, Rewi Matiu John Gemmell, 17, Kent Russell Burns, Gabriel Williams, Anthony Richard Lloyd, Mark Peter Hagen and Daniel Martin Cutbill, all 18, pleaded not guilty and went on trial in the High Court at Gisborne last month.

Castles gave evidence against the six at their trial. All six were found guilty of sexually violating the student. Hagen, Lloyd and Gemmell were also found guilty of attempted sexual violation. Williams, Burns and Cutbill were acquitted on that charge but Williams was found guilty of an alternative charge of indecent assault.

The six will be sentenced on Friday.

Ms Crutchley said the sentencing judge had erred in setting the starting point for Castles' sentence at five years.

The Crown did not challenge the judge's decision to give Castles a discount of 50 per cent for his early guilty plea and his co-operation with investigating officers.

The attack on the boy was akin to rape, she said. It had been an expression of power over another person, which had the effect of humiliating and degrading him.

"What he has expressed, the effect on him, is the same as many female victims of sexual violation express."

It was not relevant whether Castles' motivations had been sexual, what was relevant was the impact on the victim, she said.

Defence lawyer Steve Manning posed the question of what sentence would have been imposed if the incident had involved a woman victim and seven female attackers.

There had been a degree of perceived unfairness in the way the Crown had gone about lodging the appeal. Castles had worked closely with the Crown and police leading up to and during the trial yet no one had told him it was to be lodged.

The case was unique and there were no sentencing precedents for the judge to compare with, he said.

The 2 1/2-year sentence was "very sensible" considering Castles' co-operation and guilty plea. It was also severe enough to send a clear deterrent message to others.

Justices Blanchard, Hammond and Salmon reserved their decision, saying they would try to deliver it before the other six were sentenced.