A parent from the school, where fees range from $23,000 to $37,000 a year, told the Herald the younger boys allegedly had their faces covered with pillowcases and were screaming in fear.
The father said parents had been horrified to learn of the incident.
"A group of Year 12 boarders in School House marched a group of the Year 9s outside in the middle of night with pillowcases on their heads and their hands behind their backs, like some sort of prisoners of war," he said.
"Apparently the idea was to scare them. But as you can imagine the young boys were disoriented, they couldn't see anything.
"They were petrified. They thought they were going to be beaten up. The boys were beside themselves in horror."
A senior member of the group allegedly assaulted one of the younger students "by hitting him on the back and legs", the father said.
"The poor kid didn't see it coming - he couldn't see anything. He was traumatised and called his mother, who came to collect him from the school at 3.30am."
Parents understood she planned to lay a police complaint in relation to the incident, the father said.
New headmaster Mike Leach, in only his third week at the helm, said last night he was unable to answer specific questions about the incident, as it was still under investigation.
But he said he was "extremely disappointed" by the information he had received so far.
"The college will be uncompromising when it comes to bullying."
Mr Leach understood that a parent had collected a student from the school following the incident.
He said King's become aware of the incident on Thursday morning after a student approached the teacher in charge of School House.
"We're not happy with the incident of bullying [if] it happens at any stage and we will deal with it."
The senior students involved would not be back at King's until they had met the school board's disciplinary committee this week.
The junior boys affected would be offered counselling, and the parents of all the students involved had been notified, Mr Leach said.
The father said of the incident: "It's unacceptable. We pay a fortune ... to send our children to the best school in the country, only to have our sons bullied like this."
King's has been rocked in previous years by the deaths of three students and several cases of alcohol abuse.
When asked about the supervision of boarders, Mr Leach said: "It was the middle of the night [when the incident allegedly occurred] and the staff are not always awake 24/7. There is normally a tutor or a housemaster through the night that if students have issues they can come to them."
Secondary Principals' Association president Tom Parsons said hazing or similar initiations were "not only politically incorrect, but mentally and physically unsafe".
"Unfortunately it has been with us for a long, long time. But the time has passed now, there is no room for it, and every principal will agree.
"It happens, unfortunately right throughout our society -- the All Blacks have had it, regional rugby teams have had it. But it is not healthy."
Mr Parsons said he had no doubt that the management, staff and students at King's would be mortified the incident had happened. It was likely the senior students involved would recognise their mistake, too.
"These things will continue to happen, and it is really, really unfortunate. And I am sure it will be dealt with appropriately.
"The issue is not necessarily how the kids behave, but what remedial action staff will take."
As of this morning Counties Manukau police had not been contacted by the parents of the alleged assault victim.
The incident is being discussed on social media, with one Twitter user responding to questions from a former student by calling the situation a "massive overreaction".
He claimed the three Year 9 students involved were "laughing about it last night, even the one who called his parents".
"And two of them were giggling while it was happening too."
King's College
*Independent decile-10 day and boarding school in Otahuhu
*Founded in 1896
*931 students
*Boys Years 9-13, girls Years 12-13
*Annual fees $23,000 to $37,000
Source: kingscollege.school.nz
