The Human Rights Commission is aware of policies barring pupils who want to take same-sex ball partners. It says excluding someone on the basis of their sexual orientation or gender is potentially unlawful.
Rainbow Youth, an education service for non-heterosexual young people, is holding an alternative ball next month "where it doesn't matter who your date is or who wears a dress".
The group declined to reveal the location for fear of attacks.
Rainbow education officer Serafin Dillon knows of four Auckland colleges that do not allow same-gender ball partners unless pupils sign contracts stating their sexual orientation.
She would not name the schools but said their policies were discriminatory and breached the Bill of Rights.
"If this was in the workplace it would be discrimination and it would be unheard of. But because it's a school they think they can somehow get away with it."
In 2001, then-education minister Trevor Mallard said after a similar controversy at Westlake Girls High School that he opposed schools barring same-sex partners.
Westlake pupil Tammy Webster, 18, said she had been allowed to take a female partner to this year's ball without signing any contract.
Schools that barred same-sex partners were discriminating against gay pupils or those who only wanted to take their best friend.
"It fully gives the impression that it's wrong to take a same-sex partner and I don't think that it is."
Rongotai College pupil Joshua Wright, 16, said policies that discriminated against gay pupils were cruel and unfair.
He planned to take a male ball partner. "If [the school] ask me to sign a contract, I'm going to say 'no' and just go anyway."