Conservative leader David Cameron has challenged the government's plan to have councils claw back budget surpluses from schools in England.
Some head teachers are discovering their savings are at risk
The government intends to require local authorities to redistribute to other schools 5% of all surplus balances.
In Commons questions, Mr Cameron said head teachers should have autonomy over their spending and saving decisions.
Prime Minister Gordon Brown said the surpluses - totalling £1.7bn - should be used to improve education standards.
He said the current consultation on this would end over the next few days and a decision would be announced.
Mr Cameron said: "Why do you think you know best how to spend this money, rather than the head teachers?"
He added: "There is nothing more important for our future than raising schools standards through real reform.
"An important part of that reform is being prepared to give schools real freedom and autonomy, including over their budgets.
"It would fly in the face of such autonomy to punish schools which budget carefully."
He characterised the government's plan as being "to confiscate 5% of the surpluses of good schools who plan carefully".
Mr Brown said many schools planned to use the surpluses and would be able to do that.
He said: "We are consulting on how we can best use these surpluses for the benefit of children's education and the secretary of state for children will report back in the next week.
"We are determined that money goes to the pupils and the parents to improve their education."
Although the proposal was announced some time ago, many head teachers are only now realising what is intended.
Mr Cameron said: "This is a serious issue for schools up and down the country. Let me quote some of the head teachers.
"They say it is unjust, an ill-conceived idea. One said it undermines governors' authority. Another that it destroys the trust between schools and government."
The Tory leader said: "Why don't you just scrap this consultation let the schools keep their surpluses?"
Mr Brown told him: "We are consulting on how we can best use them to the best effect for pupils and teachers."
The only reason the surpluses existed was because of the payments the government made direct to schools, he added.