Mr Uffindell will stay in parliament and the opposition National partyroom after leader Chris Luxon declared the 38-year-old a "changed man" since the 1999 assault.
As a 16-year-old, Mr Uffindell raided a 13-year-old's dorm room and beat him, allegedly with bed legs, leaving him bruised and traumatised.
He was expelled from Auckland's exclusive King's College and chose not to apologise to the victim until last year.
Since the act was revealed by news outlet Stuff, Mr Uffindell has conducted a media blitz, appearing on every major television and radio station to express remorse.
"It was my action. It was my fault. It's my responsibility. I own that and I'm trying to live my life as a responsible adult," he told TVNZ's Breakfast show.
In an extraordinary press conference at Wellington's parliament house on Tuesday, Mr Uffindell said he was a teenage bully who committed other bashings - though none of that severity - accepting he had committed a crime.
He also admitted dishonesty by not telling National preselectors or voters of the assault.
"In hindsight we would have been better to get that out earlier," he said.
"I wasn't a great person at high school. I'm not proud of that person. I was a bully."
Mr Uffindell also faces charges of hypocrisy, given he campaigned on law and order and spoke in his maiden speech of a "growing culture of lawlessness and lack of accountability".
"Absolutely I can see how it's been damaging to me and it has also been damaging to the party," he said.
"I really want to stay on as the member of parliament for Tauranga. I want to use my position to do good."
Mr Uffindell's victim - speaking anonymously to Stuff - accepted his apology last year but came forward when he saw Mr Uffindell running for parliament and "felt sick".
Drawing a line under the issue, Mr Luxon said his colleague "has my backing and he has my support".
"Sam Uffindell has changed from the 16 year old teenager that we saw that was expelled from school 22 years ago."
In the morning, Mr Luxon said he didn't think Mr Uffindell's actions constituted a crime, saying "we're not talking about criminal behaviour" but later reversed his position.
Police were not involved at the time of the assault.
Mr Luxon said he sympathised and wanted to "centre" the victim but did not agree Mr Uffindell should resign if that is what the victim wanted.
"My thoughts are very much with the victim. I can only imagine the hurt, the pain, the suffering, the impact of this event has had on them," he said.
"There is no place for violence in New Zealand. No context, no setting, no excuses. We abhor it and we don't condone it whatsoever."
Prime Minister Jacinda Ardern declined to call on Mr Uffindell to resign, instead calling on Mr Luxon to show leadership.
"When it comes to the management of our candidates, our members, our MPs, it does come down to individual party leaders to take responsibility," she said.
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