Most schools can spread the changes across many year groups but the effects are more concentrated in intermediates which cater only to Year 7 and 8.
As well as that, under the Budget moves, the 1 to 120 ratio that intermediates providing technology subjects (art, cooking, woodwork etc) get, was assigned to the student's contributing school, rather than the provider school.
That affects 30 per cent of intermediates.
Mr Key said that over the last 10 years, the number of teachers had "massively increased" but there had been a modest increase in rolls and the teaching outcomes had remained the same.
"International research supports what the Government is doing which is to increase the quality of teaching, not massively increase the number of teachers... It will be irrelevant whether your child is sitting in a class that has 15 children or 16 children. It will be highly relevant whether that teacher is a good quality teacher."
Labour leader David Shearer said Labour would reverse the changes to class size ratios and would review the changes National is making to teacher qualifications.
"Quite clearly smaller class sizes are better for children's learning. Everybody knows that."
Asked how he would fund the changes, he said about 40 per cent of the $43 million in savings National expected to make from the changes had not yet been re-allocated for spending elsewhere. He said Labour would initially return class size ratios to the level they were at before National's changes and then try to reduce them further.
"We've always believed the smaller you can have class sizes the better because kids get individual learning. Our ideal would be to continue to reduce class sizes as financial conditions allowed."
Labour would also review the changes National was making to teacher qualifications.
"I think some would argue that in primary schools, for example, a three-year primary teaching qualification might be a superior thing than a one-year post-graduate qualification."