"They are all angry. They have every right to be," he said.
Proposed cuts that will mean larger class sizes and fewer staff had incensed everyone, he said.
Education Minister Hekia Parata announced new ratios as part of the Budget without realising that, among the 244 adversely affected schools, some could lose up to nine teachers.
Last week she hastily limited lost positions to no more than two full-time equivalents over three years.
But work is continuing on the revisions.
Ms Parata has rejected a call to meet education groups that have united in opposition to the funding proposals, which will increase class sizes.
Instead, she says, she will try to defuse their anger and meet the organisations individually this week.
An alliance of unions, principals' organisations and the Schools Trustees Association met in Wellington on Tuesday, to call for a reversal of the proposed student-teacher ratios and immediate discussions with the Government. Ms Parata declined the offer of a meeting, saying last night it was something she could consider down the line.
But she suggested there was room for negotiation on provision of technology teachers for classes such as art, music, cooking and woodwork.
Mr Withers labelled the proposed changes an "absolute disaster".
"It's like some terrible experiment being carried out by the Government. But what's really horrifying is that the Government and Ms Parata were driven initially by advice from Treasury ... they are the ones with muscle," he said.
"Why would you want Treasury's advice after the demise of our finance companies ... I mean, why would you?"
Earlier, Wanganui Intermediate School principal Charles Oliver said the changes would mean the loss of teaching positions and the complete removal of all specialist teaching.
"If this goes ahead, we will be unable to staff our programmes in technology [cooking, metalwork, robotics], art, science or music."
It was those programmes that made intermediate schools special and different.
Parents and teachers were not prepared to accept the changes, Mr Oliver said.
Ms Parata had reiterated that half of schools would gain staffing entitlements.
Savings from class size changes are to be used to develop an appraisal system to help lift and reward teacher performance.