Green Island School principal Steve Hayward said he was notified this week his school's decile would change from 4 to 6, which meant the school would receive about $10,000 per year less in Government funding.
''It's unbelievable.
''There's been no new housing developments, no change in our circumstances ... I don't know why this is.
''You can't say there's a new housing development here, or there's a heap of new people living here, or the incomes of people have gone up.
''There's nothing in particular that stands out to me as having changed in Green Island.''
Mr Hayward said the school would now have to tighten its belt and find ways to make ends meet.
''$10,000 per year - that money's got to come from somewhere.
''This is real money for teacher aides and support programmes for children.''
He was angry that early in term 4, the school set its budget for next year, only to find the new decile announcements had changed its finances.
''We have to go and redo our budget now. It's not a simple exercise balancing the books.
''I'm pretty angry about that and so are a lot of other schools.''
He said the school planned to apply for a review.
Green Island School was not the hardest hit. St Clair School principal Richard Newton said his decile went from 6 to 8, which meant his Government funding would be cut by $18,000 a year.
Big Rock School principal David Grant said his decile was altered significantly - from decile 5 to 8 - but because his roll was smaller, his funding fell by $4500 a year.
There have been unconfirmed reports some South Island schools' deciles had gone up four ratings.
Mr Grant and Mr Newton said they would deal with the cuts by managing things ''a bit smarter''.
''It won't affect the education of the kids. We'll find a way around that,'' Mr Grant said.
''If the kids start missing out, we may as well close the doors.''
Schools expecting reduced funding have been told by the ministry there would be an 18-month transition period for schools to adjust their finances.
The Ministry of Education would not comment on individual schools yesterday.
However, its website says the ministry uses decile ratings to allocate additional funding, which enables schools to overcome barriers to learning faced by pupils from low socioeconomic households.
A decile is a 10% grouping. There are 10 deciles and about 10% of schools are in each decile.
Decile 1 schools are the 10% of schools with the highest proportion of pupils from low socioeconomic communities, whereas decile 10 schools are the 10% of schools with the lowest proportion of these pupils from poorer homes.
The lower a school's decile rating, the more decile-based funding it gets to provide additional resources to support their pupils' learning needs.
A decile neither indicates the overall socioeconomic mix of the pupils attending a school, nor measures the standard of education delivered at a school.
Deciles are calculated by examining five socioeconomic factors in a school's community: household incomes; occupation; household crowding; educational qualifications; and income support.
Schools are ranked in relation to every other school for each of the five factors and receive a score based on the percentile they fall into. The five scores for each school are added together to give the overall standing of the school in relation to all other schools in the country.
Schools are then placed into 10 groups called deciles, each having about the same number of schools.
