Tangiteroria School located 32km south-west of Whangarei, which caters for years 1 to 8, increased from a decile 5 to 9, meaning a significant cut to funding.
Principal Megan Tebbutt said the move came as a huge shock because she did not believe there had been any change in the socio-economics of the community.
"It really takes it off our students," Ms Tebbutt said. "We can't spread it any thinner".
Ms Tebbutt declined to say how much the school was likely to lose but figures around new school funding rates were expected to be released by the ministry in mid-December.
The Northern Advocate understands there are a number of schools in Northland which plan to appeal their new decile, including Tangiteroria School.
"Our first reaction was that someone had put in the wrong data because there is no reason for it."
Deciles rank schools on a scale of one to 10, according to the extent the school draws its students from low socio-economic communities.
Decile 10 schools are the 10 per cent that draw least from these communities, while decile 1 schools are the 10 per cent that draw most from them. The lower the decile the more funding the school receives from the Government.
A decile does not indicate the quality of a school and does not take into account the achievement of students, quality of staff or leadership at the school.
Ms Tebbutt said when she met with a Ministry of Education representative they were unable to explain why this change had occurred.
The only other schools in the region to make such a jump was Arapohue School, which went from decile 3 to 7, and Maungakaramea School, which dropped from decile 9 to 5.
Deciles are calculated using data from each census, meaning they are reviewed every five years.
However, this time it was left seven years, as the Christchurch earthquakes caused one census to be postponed.
Te Tai Tokerau Principals Association president Pat Newman said the new deciles showed Northland schools were the poorest in the country.
Allocating funding to schools according to deciles has stirred controversy lately with even Minister of Education Hekia Parata calling it a "blunt instrument".
Mr Newman said: "We know that it is not the greatest but it's far better than any system we've had in the past."
The higher funding rate for schools that received a lower decile would apply from the start of the 2015 school year. However, schools receiving less funding would have the new funding rate gradually phased in over 18 months.
Nationally, about a third of schools remained the same decile, a third went up and a third went down.
