In January, the school decided to bring in a "mould doctor" - an environmental scientist specialising in mould and air quality.
The test results confirmed their suspicions and, with support from the Board of Trustees and the Ministry of Education, the school removed staff from the building.
"These results were quite damming of the quality of air staff had been working in," he said.
Temporarily moving the staff was a "massive inconvenience", he said, but the school would ensure there was no disruption to parents or students.
The library will be relocated to an empty classroom and the school was establishing a mobile system "to bring books and readers into the classrooms".
The administration building houses six staff as well as the staffroom, reception, administration offices, boardroom, principal's office, staff toilets, stationery storage, special-needs toilet and medical room.
The school and the ministry now had to decide whether to fix or demolish the building, Skilton said.
The ministry's head of education infrastructure service, Kim Shannon, said the ministry was working to support the school.
"We are assisting to ensure the administration staff can continue their day-to-day work," she said. "We are helping the school arrange for equipment from the administration block to be cleaned."
She said schools received funding to maintain and develop their property, which included repairing any leaks or dampness.
"We take health and safety in our schools seriously and we will act immediately if a school alerts us to concerns. We have been working with the school on this issue since late last week when they first alerted us to the problem."
She said weather-tightness testing was carried out on the administration building in 2010 and no issues were identified at that time. Remedial work at the school that year did not include the administration block.
Toi Te Ora Medical Officer of Health Phil Shoemack said black mould could cause infections but working in a damp building was likely a bigger contributor to staff illness than the mould.
"I hope they do more than just get rid of the mould. They need to get rid of the damp."
Tauranga City Council moved more than 500 staff out of its Willow St administration building in 2015 following the discovery of black mould.
The building has been partially demolished with planning under way for a $23.2 million replacement.
Black mould
What is black mould?
Toxic black mould or black mould is the common name for Stachybotrys chartarum. It can grow in houses and produce mycotoxins that pose health risks to humans.
What symptoms are associated with black mould?
Commonly reported respiratory tract symptoms include nasal irritation, burning and congestion, cough, wheezing, chest tightness and dyspnea. Central nervous system manifestations include headaches, irritability, lightheadedness, sleeping difficulty, concentration problems and mental fatigue.