There's an inevitable outcome when any policy with the potential to go seriously wrong is badly prepared and poorly presented.

It's a train wreck, and that's what has happened to the government's decision to increase class sizes and cut teacher numbers.

It might have gone badly wrong even with the best planning and presentation, but ministers don't seem to have even considered the possibility.

So they're in damage control, fighting teacher unions and trying to deal with opposition parties which have been given another gift-wrapped opportunity to seize the initiative, set the agenda and gather public support.

National's support partners, United Future and the Maori Party, are acutely aware they're going to be caught in the crossfire.

They want to get out the way, and they're looking for escape routes.

The rationale for the decision to standardise class sizes at 27.5 pupils for each teacher - they currently range from 23 to 29 - is simple.

It's a belief that a bigger class with a good teacher delivers better results than a smaller class with a poor teacher.

The government figured out that by making a small change to the ratio it could save $43 million a year, which it could use for teacher development.

Education Minister Hekia Parata says she has international research and evidence which proves her case beyond any reasonable doubt, but her policy went off the rails so quickly it's all been lost in the debris.

The government seems to have failed to perceive how quickly and easily opposition parties would be able to rip the policy apart.

Labour, the Greens and NZ First don't have to even think about the validity of evidence that supports the government's case.

For them it's really simple. Bigger classes and fewer teachers - how can that be good for education? Everyone knows children get more attention in small classes, parents should be really worried and if they aren't they're going to be told they should be.

Teacher unions, articulate and highly organised, are difficult for any government to deal with.

Fighting this fiasco, they've forged a formidable joint command which presents one of the most serious challenges the government has faced.

The government's problems started with a Parata press statement which buried the ratio change and did little to explain it.

Teacher unions were free to work out their own figures, and those were that some schools could lose up to 10 teachers.

Parata said 90 per cent of schools would lose or gain one teacher, while the other 10 per cent would "find it more difficult".

That didn't appease the unions, so the next day she gave an assurance no school would lose more than two teachers.

Prime Minister John Key said that would mean the government would save "a bit less" than the $43m it had planned for.

The next day a contingency fund was discovered and Parata said the $43m would stay intact.

Parata's back-tracking hasn't done her any good.

The unions say no school can afford to lose any teachers and the policy has to be scrapped.

They're getting the message across to parents, and that's what the government should be really worried about.