But rector Richard Hall believed the letter about the May 20 event had been misunderstood.
The letter, sent to parents on April 21, said the ticket price had been set at $195 for a double ticket and included a full buffet meal and dessert.
"There are no single ticket sales. It is not appropriate to double up to get around the single ticket rule.
"Please be aware that we have budgeted this event on 150 people (75 double ticket sales).
"If we do not meet this number, we may need to cancel ...
Parents/boys may receive a refund after all costs for venue hire, music and food deposits have been covered."
The letter did not go down well with many parents, who vented online.
One said the school’s actions were "discriminatory".
"It is beyond belief that in a society where we are welcoming and inclusive of LGBTIQ persons, we would exclude someone just because they are unable to attend with a partner.
"It is not our differences that divide us. It is our inability to recognise, accept and celebrate those differences."
Others questioned how the "rules" fitted with Ministry of Education principles and guidance.
"Seems to me, money is more important than participation.
"Is this discrimination based on sexuality? Pretty sure that's illegal."
Others also expressed concerns about any refunds.
Mr Hall said the letter had been misconstrued.
"It was never an issue regarding gender or identity.
"The boys were told in person that they could bring a partner of the same gender or identity.
"That’s been misconstrued from the start."
He said a second email was sent to families a week and a-half ago, clarifying the situation.
However, he said pupils were encouraged not to attend as singles.
"What had been happening last year and the year before was that boys had been going as friends, paying for a double but sharing the cost.
"It’s not appropriate for this kind of function.
"It’s a school formal where traditionally you bring someone from the outside.
"I said if you want a function with your friends, let me know if that’s the function you would like."
He said no-one ever contacted him to take up his offer, so he took that to mean they wanted a school formal.
"By having groups of singles turning up, you actually change the culture of the event, where you bring someone from outside the school organisation into the school, you welcome them in, you look after them, you have a dinner together, you dance together, you socialise, you make friends — that’s been the whole purpose of all the 21 events I’ve done.
"That was explained to the boys and they have not come to me in any numbers, in any way, to say I’ve got this wrong."
Mr Hall did not believe there were any issues around cancellation of the event and the return of money paid for tickets.
"This year, I’m happy to say, we’ve got the ticket sales and it will be going ahead."
