Having found him guilty in May, a magistrate on Thursday sentenced the boy to nine months' detention.

She set a two-month non-parole period, making him eligible for parole on October 25.

"The young person has shown no contrition," the magistrate said.

"(The victim) was required to come to court and relive the offences. He, in effect, placed responsibility for the offences on her."

The offender and the victim cannot be identified for legal reasons.

The attack occurred on a mattress in the home's living room, after the party had quietened down and other attendees had gone home or retired to bedrooms.

The boy's lawyer had suggested it was noteworthy that the girl had her phone next to her during the attack and hadn't called out to others in the home for help. The magistrate dismissed that.

"There is no right way to deal with the situation and no right way to act, and I draw no criticism or draw no conclusions (upon her)," she said.

References of the boy's otherwise good character and supportive family and social ties mitigated the sentence, with the magistrate accepting his prospects of rehabilitation were "strong".

In addition to the custodial sentence for penile rape, he will be on probation until August 2023 for acts of digital rape, oral rape and choking committed during the attack.

The girl didn't like her attacker but was heavily affected by alcohol when the attack occurred.

She has described the boy as "a monster" who'd left her "ashamed and irreversibly damaged" and struggling with nightmares, psychological issues and low self-esteem."

He violated me and he took away my virginity," she said in a statement read by lawyer Michael Bradley outside court.

"He took away my confidence, my mental health and the healthy life I led before the attack."

The girl's family was now considering suing the NSW education department for the high school's alleged failures to address warnings about the boy prior to the attack, Bradley said.

They called for the department's report into the school's response to be released publicly.

"What happened to me is happening to far too many girls in Australia," the girl said in the statement.

"The untold number of rapes and attacks on Australia's schoolgirls needs to stop."