The source viewed the short video clip the weekend before last.
They learned about the video through an acquaintance who attends the school, who had also viewed the footage.
The boys and the teacher in the video were identifiable, the source said.
The group chat on Snapchat where the video was shared has now been deleted.
However several sources believed the video along with a text message between the teacher and student have been passed onto police as evidence.
It is understood the video sparked the school's investigation, along with a text sent from the teacher to a student.
There are now questions about how long the school knew about the allegations before it sent a letter to parents and caregivers last Friday.
Minister of Education Chris Hipkins confirmed today police were investigating the incident after requesting an urgent briefing from the Ministry of Education last night. Oranga Tamariki is also involved.
Police today refused to confirm any details and said it had nothing further to add to a previous statement saying it could not respond to queries seeking to establish whether specific individuals or organisations are, or have been, under police investigation.
The Ministry of Education deputy secretary sector enablement and support Katrina Casey also stood by her earlier statement that she understood the allegation had been referred to police and Oranga Tamariki.
Casey said the ministry would provide support and guidance to the school if needed, but would not confirm today if it was required.
In a letter sent to the school's parents and caregivers on Friday, principal Wayne Hegarty and board chairman Sturrock Saunders said they wanted to update the school community about a "sensitive matter" following concerns being raised about the conduct of a teacher at the school.
"As the matter was serious and needed to be worked through carefully, we immediately sought advice from the Ministry of Education and New Zealand School Trustees Association," the letter said.
"Oranga Tamariki and Police are investigating and have made contact with those directly impacted."
The school was now acting in a liaison and support role, as well as managing its own employment process.
The school was aware it was "disconcerting" for its school community and said its key priority had been "to support and protect the privacy of any students affected by this matter and liaise with these students and families in the first instance".
It was also being careful not to "prejudice or pre-empt any official processes".
"We do hope that you also trust that we are doing everything that is needed at this time and comfortable that we will update you as we can, but this may not be for some time as it will depend on the processed being worked through by the agencies involved," the letter concluded.
Saunders confirmed in an earlier statement to media that the school was working through a matter relating to a staff member. The staff member was immediately put on leave.
The school has not responded to additional questions put to it by the Herald.
