This will result in a big funding drop for the school and will likely mean increased reliance on parent fundraising and donations.
Mr Magner said it was too early to tell how much the cut would be.
"The funding cut will not happen suddenly and we have in some ways been expecting it, my understanding is that it will be gradual over an 18-month period."
Kathleen Mendes, whose daughter Anahara Mendes, 6, attends the school, said she could see both pros and cons to the decile change.
She said the higher decile would appeal to some parents, but would put more financial pressure on those whose children attend the school.
Ms Mendes said the school already did a lot of fundraising, but she would not mind paying a little bit more in donations as she believed the school had been great for her children.
Mt Albert Grammar School headmaster Dale Burden said it had been a relief to be told his school would stay at decile 7.
Other schools that have been told their decile will not change are decile 3 One Tree Hill College and decile 7 Three Kings School.
Frances Nelson, principal of decile 2 Fairburn School in Otahuhu, said she had been given a verbal indication of the school's new decile rating, however she did not wish to comment until she had received it in writing.
After the last Census Fairburn was promoted from decile 1, leaving the school with a $70,000 funding shortfall.
Decile 4 Mt Roskill Grammar and decile 6 Blockhouse Bay Intermediate told the Herald they had not yet received notification of their new ratings from the ministry.
A spokeswoman for the ministry said a full list of all decile changes would be released at some point after next Thursday, when they had finished talking to schools about any changes.
Graham Stoop, the ministry's head of student achievement, said roughly one-third of schools have moved to a lower decile rating, one-third have moved up and one-third had remained the same.
"A total of 800 schools have moved to a lower decile rating, while 784 schools have moved to a higher decile rating. For another 822 schools, there has been no change," he said.
He said any school was entitled to seek a review of its new decile rating if it believed the rating did not accurately reflect the socio-economic circumstances of its students.
Decile ratings
• Ellerslie School changing from 7 to 9
• No change for Mt Albert Grammar, One Tree Hill College and Three Kings School
• Mt Roskill Grammar and Blockhouse Bay Intermediate still waiting for notification
• All schools will be informed by November 20