Outgoing AEU Tasmania branch president Leanne Wright said she was wary of any change to improve retention rates as there was no evidence to prove that it would be successful.

She said the union had asked for evidence from the Liberals last year and was still waiting.

``What evidence do we have that this change will have any benefit for students in schools?'' Ms Wright said.

``There's no evidence. It's a waste of our taxpayers' money, it's an offence that our children are experimented on like this, such as what happened with Tasmania Tomorrow.''

She said politicians had a habit of trying to find simple solutions to complex problems and schemes like Tasmania Tomorrow was proof that such things did not work.

Ms Wright said the focus should be on how learning programs could be improved to cater for the independent needs of students.

Opposition education spokesman Michael Ferguson said Liberal education policy was about increasing year 12 completion rates which were the worst in the country.

He said school capacity figures showed that there was under-utilised space in many high schools, particularly those in regional areas, where the policy would be rolled out first.

Figures worked out by The Examiner, and published yesterday, show a small number of regional schools could accommodate an extension.

Mr Ferguson said colleges would not close.

``As is always the case we will provide full costings prior to the next election and after the current consultation stage with school communities which we have commenced,'' he said.

Acting Education Minister Brian Wightman said it was flawed policy, that 18 of 30 schools would need to be rebuilt and the funding set aside was not enough.

``Retention is very important . . . and we will continue to work to improve retention in Tasmania,'' he said.