Aged 16 as a Year 11 student at the Auckland boarding school, Uffindell and three others reportedly jumped on the boy and began beating him with what was believed to be unscrewed wooden bed legs, according to Stuff.
It was reported the now MP for Tauranga apologised to his victim 22 years after the attack and nine months before he revealed his political aspirations.
"It was one of the silliest, stupidest things I've ever done. I really regretted it, I do really regret it still," Uffindell said to Stuff.
Police were not involved but he was reportedly asked to leave King's College, finishing his schooling at St Paul's Collegiate in Hamilton.
The NZ Herald is seeking comment from Uffindell.
The National Party is yet to comment. It was reported the party had been made aware of the incident.
The victim, who was not named, told Stuff Uffindell contacted him through a mutual acquaintance in July last year to apologise, which the victim eventually accepted.
"But then a few months later I sat down to watch the news on the couch with a beer and there he was, running for Parliament," the victim said. "I felt sick."
Uffindell had reportedly not mentioned his political intentions during the interaction.
The incident reportedly occurred in 1999 on the last night of term inside one of the King's College boarding houses.
The victim, then 13 years old, had been in his dorm room bed after lights out, when four older boys reportedly came in, jumped on him and began beating him.
It was thought the boys had been using wooden bed legs unscrewed from their dorm, Stuff said.
At a post-cabinet press conference this afternoon, Prime Minister Jacinda Arden said the conduct of candidates and MPs was an issue for the party involved.
Asked about any hypocrisy given Uffindell has been campaigning for stronger action on gang violence, Ardern said as leaders "we need to be accountable for our members' conduct".
"It is up to the National Party and [leader] Christopher Luxon how they conduct their own affairs."
Deputy Prime Minister Grant Robertson said violence and attacks on young people are something people should all be very concerned about.
Max Key, a former King's College student and son of former Prime Minister Sir John Key, said in 2016 that he was "bullied quite a bit at school so I found that pretty hard, growing up".
However, he said, King's offered him support, and bullying was not tolerated.
"If you were caught bullying, it was suspension."
In 2014, allegations of hazing at King's also emerged.
