One boy was reportedly hit during the night-time hazing incident.
Leach said the senior boys each told the school board a similar story about the incident at 3.30am on Thursday, October 23.
"They felt it was a prank and it went wrong," said Leach.
He said the senior students, who have been suspended until the end of the year, would still be able to sit their external exams.
The board did not want the incident to impact on future study and was keen to work around the suspension so the boys could sit university entrance exams.
They had even been advised to look at applying for aggregate passes for subjects if they had become unduly stressed by the disciplinary process. "We don't want to trivialise the event and we won't tolerate bullying but we understand this is university and their future."
Leach said the three Year 9 boys had returned to the college's boarding house after Labour Weekend and intended to carry on as boarding pupils at the private school.
"All three have indicated they will be back at King's and we have been in constant contact with their parents.
"All the boys have had opportunity for counselling and taken it and we are discreetly touching base every day in case they need any more support," said Leach. The headmaster said he had watched the young boys playing cricket and tennis at school yesterday.
He said the range of conditions to re-enrol at King's varied for each of the suspended boys.
Each was required to go through some form of anti-bullying training. That included measures to help young men stand up to peers and be advocates for other students in the face of bullying.
"They will also be expected to do community service," he said.
They would use resources from the New Zealand Peace Programme, the Ministry of Education and the police.
The dormitory incident had meant King's would look at reviewing its safety programme schoolwide.
Leach, who is just three weeks into his new headmaster job, was unable to say if there was a bullying culture at the school, but said this episode provided an opportunity to examine the culture and delve more deeply.
"In any school I have been in the first priority is that students will not learn unless they feel safe.
"Parents trust that when their children walk through the gates that you will look after their children.
"That's a mandate that boards, teachers and especially principals need to take on board and fulfil that responsibility."