Labour is calling on National's coalition partners to force the government to scrap its plan to increase class sizes and cut teacher numbers.

There's a battle raging around the policy with United Future and the Maori Party caught in the crossfire.

Their agreements with National will force them to vote for it because the policy is part of the budget - a confidence vote - but neither is happy about the way they've been treated by ministers.

They both say they weren't properly briefed on the policy before last week's budget, which apparently breaches the "no surprises" clause in their agreements.

"We are concerned that sufficient detail was not shared with us and we will be advising the prime minister of our concern," Maori Party education spokesman Te Ururoa Flavell said on Friday.

United Future leader Peter Dunne says he has serious concerns about the policy.

"The information we had wouldn't lead you to conclude this sort of situation was going to explode," he said.

"I'm obviously very frustrated."

Mr Flavell isn't directly criticising the policy but says he's disappointed the "full implications" of it weren't revealed ahead of the budget.

Teachers and parents are already rebelling against the plan to save $43 million a year by increasing class sizes and using it for teacher development.

Labour leader David Shearer says United Future and the Maori Party must use their political muscle.

"They must use their power to force National to drop the plan completely, not just tinker with it," he said.

"They have an opportunity to stand beside Kiwis on this issue ... parents, teachers and children are overwhelmingly opposed to it."