His daughter, who has attention deficit hyperactivity disorder (ADHD) among other issues, was struggling in mainstream education and had been expelled from one school, before she was accepted into Salisbury.
"She's incredibly happy at Salisbury, they're an amazing school," Mr Clayton said.
"They really turned her life around completely, and the last thing she needs is to be removed from there because if he was exposed to a high school again, it'll wreck her. She's functioning three or four years behind where the other girls are, and it would just destroy her, she wouldn't cope at all."
Salisbury School was an "incredible facility which has had generations of success", he said.
If it closed "my daughter has nowhere to go", he said.
"It's the first place where she's ever been happy, she loves it."
He railed at the National Government's approach to special educational needs, and said the Ministry had been attempting to close Salisbury School for years.
"This Ministry has shut down all the other schools that she could have possibly gone to. I'm a high school teacher so I'm very aware that as a high school organisation we do not cater for the bottom 5 per cent at all. So where do these girls go?"
The $214,909 per child the Education Minister Hekia Parata cited yesterday as reason for questioning Salisbury's economic viability, was "a situation [the Ministry] deliberately created", Mr Clayton said by changing the admissions criteria.
"In my opinion, they've set this up from the beginning to close it, and I'm just disgusted with them," he said.
"What kind of society do we live in where we're always prioritising money over the welfare of the citizens?"
Ms Parata denied she had been intent on closing Salisbury School, or that she had tweaked the process to ensure its falling roll.
"Salisbury's roll has fallen because the introduction of the Intensive Wraparound Service has provided parents of girls with high and complex needs with an alternative to residential schooling that is proving popular," she said.
"All parents of children prioritised for intensive, personalised care still have the option of residential schooling, but an increasing number are opting for localised support that enables them to keep their children at home.