The Government says it is committed to working with schools to lessen the impact of bigger class sizes.
Principals say the change in student-teacher ratios could spell the end of technology teaching in intermediate schools.
From 2013, the teacher-student ratios for Years 2 to 10 will increase to one teacher to 27.5 students.
Intermediate schools say they will have to get rid of many technology teachers because last Thursday's Budget abolishes specially allocated funding for them.
The Prime Minister said on Monday a small number of intermediate schools will be "overly affected" by the changes.
However, John Key says a Government working group will look at how the pressure on those schools can be reduced.
Mr Key says 90% of schools will either lose or gain a teacher as a result of the changes but, overall, teacher numbers will remain stable over the next four years.
Working group stalling tactic - Labour
The Labour Party the Government has set up the working group to protect itself from a massive backlash from parents and teachers.
Leader David Shearer says letters and emails have been flooding into the party's offices from parents and teachers horrified at the impact the Government's decision to increase class sizes will have their children's education.
Mr Shearer says the Government is using the working group as a stalling tactic to take the heat off itself.
"It's a smokescreen, let's face it. It's already made the decision and what it's trying to do now is simply stall the ultimate decision about increasing class sizes."
He believes the Government should cancel the working group and its policy to increase class sizes.