The secondary teachers' union has called meetings for next month to plot a campaign against what it calls a "government attacks on frontline teaching staff and class size".
Post Primary Teachers' Association (PPTA) president Robin Duff said cuts announced in last week's Budget amounted to "a $300m clawback of school staffing over the next five years".
"Subject options will have to be cut and area schools and junior high schools will no longer be able to function the way they were designed to operate," Duff said.
Paid union meetings were being organised for next month to discuss "staffing cuts and to develop a campaign to fight for their reversal," he said.
Changes to the funding ratios for schools were confirmed in last week's Budget, but the Government yesterday announced the full impact would not be forced upon the most-affected schools until after a three-year transition period.
Under the changes, about 245 schools stand to lose anywhere between one and seven teaching staff.
Education Minister Hekia Parata yesterday announced that funding for no more than two full-time equivalent teaching staff would be lost at any school in the next three years.
Because of the "transition" period, the Government would "clearly not" make the savings it had outlined in the Budget, which had been set down as $173.9m over four years, she said.
Also as a result of the changes, a further 765 schools will next year lose funding for less than one FTE teacher, 962 will gain less than one, 251 gain more than one and 213 have no change. The overall number of teachers in schools across the country is not expected to change much.
Duff said the loss of one teacher was a bad thing for any school, but losing two would be a "disaster for most".
"If the Government had consulted with the education sector in the first place, as it constantly claims it will do, this whole mess might have been avoided," he said.
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