Principals said the changes would directly affect children's learning.
"It's an absolute travesty, it's outrageous," said Iain Taylor of Manurewa Intermediate. "On the one hand they want to raise achievement, but on the other hand the Government is making it even harder for us to do so by these changes."
Mr Taylor said to keep subjects such as food, art, drama, information and communication technology and learning support, his decile-one school would need to lose 6.75 staff and increase its class sizes from 27 to 37.
"The minister says it's up to the schools what subjects they offer, which is absolute rubbish because we're stuck with a funding formula which stops us from funding them."
Mr Taylor said the biggest joke about the whole policy was that the Government kept talking about the under-achievement of Maori and Pasifika students, yet the specialist subjects were what drew them to school.
"So motivation will be less, and how are we supposed to address the under-achievement in the long tale when there are class sizes that are massive?"
Michael O'Reilly of Mt Roskill Intermediate echoed Mr Taylor's concern that Maori and Pasifika students would suffer the most.
The Auckland central school is decile 4 and has a very diverse population. Last year, Mt Roskill's Pasifika students were the ones who made the most progress.
Mr O'Reilly said if his school was to maintain its specialist subjects, it would need to lose up to five of its 28 teachers and increase class sizes to 34 or 35 from the current 29.
"I find it ironic that they're spending so much money on research and development and yet it's those technology classes that really ignite the children's love of and passion for tinkering and learning ... which is what New Zealand was built on, that No 8 wire."
Jonathan Tredray of Northcross Intermediate on the North Shore called the changes "devastating" and an "absolute nightmare".
The decile-10 school has worked out it will need to lose up to 10 teachers from a staff of 50 - including three teachers provided by its board of trustees - and increase class sizes to about 40 to keep its technology subjects.
"Having that many people in a class is just ridiculous. It's been proven that kids these days learn differently and they need smaller class sizes ... Students would just get lost in a class of 40," Mr Tredray said.
"If we did have to go through with this, we would have to revert to the old style of 'sit-down, shut up and learn', which is not how students learn."
Grace Dickison, 12, does kapa haka at Northcross and feels it boosts her confidence and motivates her to go to school. But her mother, Kirsten Dickison, says that if the funding is cut and a teacher is laid off, it would be "devastating".
"Grace loves it and it would be a shame to see others not be able to do it."
The Mairangi Bay mother has already had two children go through intermediate, one of whom thrived because of dance classes.
"Those intermediate years are all about the kids trying things that they wouldn't normally try because they wouldn't have that opportunity unless [parents] are actively involved in putting them into dance or music outside of school."
Mrs Dickison was also concerned about the increased class sizes proposed by schools to keep the specialist subjects, saying teachers were already stretched to their limits.
"It doesn't fill me with confidence, really. Life for teachers is hard enough as it is ... How are they going to meet the demands of more students?"
Mr Key said there would be a minimal impact for 90 per cent of schools - most would either lose or gain one teacher. However, other schools - mainly intermediates - would be "overly affected." The intermediates' biggest issue was that they would be unable to spread the impact of the ratio changes across as many year groups as other schools.
However, the Prime Minister stood by the decision to change the ratios overall, saying the money saved would go into improving the quality of teaching.
Labour Party leader David Shearer said Mr Key had underestimated the impact of the changes to class sizes and the working group was "a smokescreen", which would make no difference.
"This week, principals will be writing to parents and telling them the implications of the changes to class sizes and I think parents haven't quite yet recognised what's coming down the track."
He said two intermediate schools in his Mt Albert electorate had told him they would lose at least three teachers.
"I can't remember any Government advocating increased class sizes as the way forward in education."
TEACHER-STUDENT RATIOS
Now
* General staffing ratio of 1 teacher per 29 students.
* Specialist technology staff of 1 teacher per 120 students.
The change
* 1 teacher per 27.5 students.
* No staff for specialised subjects.
What it means
* Schools have to lose specialist staff.
* If they want to keep specialist subjects, they have to make staff cuts elsewhere and increase class sizes.