Salisbury School may mount a legal challenge against Education Minister Hekia Parata's decision to close it at the end of the year.
But the minister says the ministry went through a thorough and robust process.
Ms Parata announced her final decision yesterday, after a six-month consultation process.
The Richmond school will be closed along with the McKenzie Residential School in Christchurch, in favour of co-educational residential facilities, Halswell Residential College in Christchurch and Westbridge Residential School in Auckland.
There will be an expanded wrap-around service for children elsewhere in the country.
Salisbury has been open since 1914 and now has 62 staff and 65 students from around the country.
Its board of trustees chairwoman, Helen McDonnell, said she and staff were bitterly disappointed with the result.
"I think that now it's a statutory decision, the legal issues are the only way forward."
Lawyer Mai Chen, who provided legal advice to the school when it made its final submission on the plan, said there were questions around whether the legislative tests had been met.
The school had been told it would be closed from January, but to do so the students would need to be shifted to Halswell.
But section 146(A) 2 of the Education Act 1989 meant Halswell, a single-sex school, could not legally become a co-educational school before February 1, she said.
"They can't close Salisbury and send the girls to a facility which is still an all-boys' school."
Taking the boys away from the school grounds until the schools' legal status had changed would not be enough, she said.
There was also a concern over whether the ministry had properly advised the minister of the possible risks by sending them to a co-educational school.
The girls had teenage bodies, but mental ages of about 5, mixing with boys who were the same, and there were concerns over pregnancy, sexual contact, and sexual abuse, she said.
"There's a reason why there are a male and a female schools," she said.
To disestablish a special school, the ministry needed to provide a similar provision, which would be another school for girls in this case, she said.
But there was also the problem of timing, with any court action needing to be done before Christmas, only seven weeks away.
Ms Parata said she was not aware of any potential legal challenge, but said that the process the ministry had followed had been robust and staff had been "extremely thorough".
"That never precludes the possibility that the school undertake a judicial review."
The minister said the closure decision had been an extremely difficult one.
Current experience with wrap-around care had been positive, with learning schedules tailored to the individual, she said.
"It's not an either-or situation, it's an and-and. We will be able to double the numbers of kids."
Finding new positions for affected staff was a next step, but there would be provisions in their contracts, she said.
Representatives of the school found out about the decision when they were met at the Nelson Airport by ministry staff yesterday and given a letter from the minister.
Mrs McDonnell said the school had approached the consultation process in good faith, and had given strong evidence to back their case rather than "just jumping up and down".
"I'm hearing about the one-in-five long tail of underachievement, we have a whole school that falls into that category. The same ministry that's trying to solve that is dismantling [the school]."
With the vast majority of students at the school not from Nelson, the impact of the school closure would be felt nationwide, she said. Co-educational residential schools would put the girls at risk, she said.
"We just think it's completely putting them at risk, putting them into that situation, and it's unnecessary when they have been presented with some really sensible ideas about how this can work and actually save lots of money.
"I certainly wouldn't send my daughter down into a co-educational residential facility.
"I would have to have a huge amount of reassurance as to how that's going to work."
After the minister's initial decision was announced in August, the school had given the minister two alternative plans.
One involved maintaining two single-sex schools in their current locations but downsizing the roll and streamlining the services, governance and funding of each.
This would have saved the ministry $3.2 million, according to the school.
The other option involved having two separate single-sex schools on the Salisbury School site, with the schools being physically separate but with an integrated management and governance structure. This plan would have saved $7.3m.
The school felt it had presented a compelling argument that met the minister's purported goals around saving money and expanding the wrap-around service.
"I feel sad that we haven't been heard as a board.
"I feel sad that the voice of the community hasn't been heard, the voice of the people who signed the petition."
Does more need to be done to boost male teacher numbers?Related story: Efforts to get more male teachers failing
View births, marriages and celebrations from around the region
View obituaries from around the region