A teenager critically injured in a street brawl early on Sunday was on his way to an after-ball party organised by parents of Auckland Girls Grammar students.
Hundreds of students turned up at the private party in an old factory in Grey Lynn.
Tickets were pre-sold at $35 each and the price included alcohol, according to students who were spoken to after the assault.
The 17-year-old De La Salle student suffered multiple skull fractures after getting into a fight on his way to the party.
He was in a critical but stable condition in hospital yesterday.
The fight may have been triggered by a disagreement between students from two Catholic boys schools.
"There is some disagreement between the members of De La Salle College and past or present students of St Paul's College," said Detective Sergeant Gerry Whitley.
"We are still trying to get to the bottom of that."
Mr Whitley said the assault itself was the main focus of inquiries but police would also look into the party.
"I'm not going to get into the legalities of the after-ball. That will be something someone else will look at later on."
A student who witnessed the fight said it involved 10 students - five each from St Paul's College in Ponsonby and De La Salle College in Mangere East.
He said the victim made some comments to the students from St Paul's and a "one-on-one" fight started.
The witness said a 20-year-old former student of St Paul's came from behind and hit the 17-year-old in the head with a bat.
The student's friends then joined in the fight, as did the other students from St Paul's, said the witness.
A few seconds after being struck in the head, the 17-year-old "froze" with his fist still clenched and dropped to the ground.
St Paul's principal Denys Marra said the college was working with police and had identified a former student who might have been involved.
Mr Whitley said police had identified people they were interested in speaking to. He said the alleged weapon had been described as a large piece of wood.
Mr Marra said a large number of St Paul's and De La Salle students were at the party as legitimate ticket-holders.
"From my investigations, it would appear parents ... did charge and for that money they did supply liquor."
Mr Marra said he had visited the abandoned warehouse and said it was "beyond me" how parents could consider it for a party venue.
Auckland Girls' Grammar principal Liz Thomson said parents had organised the ticket-only party independently of the school.
She did not know the identities of the organisers but said about 20 parents were at the party.
Two years ago parents were horrified to discover several after-balls had been held at the Headhunters gang headquarters in Ellerslie.
Last year principals warned students they would face tough sanctions if they were caught going to after-ball functions.