Hard-won improvements in key stage 3 pupils' performance could be lost because of funding cuts, headteachers have warned.
The Secondary Heads Association Cymru says ring-fenced funds for KS3 initiatives have disappeared from local authority budgets, leaving secondaries short of up to pound;70,000 each. It also warned schools to expect standstill budgets at best next year.
The Assembly has allocated extra funding to boost standards at KS3 for the past four years.
This summer Welsh 14-yearolds did better than their English counterparts in science tests (74 to 66 per cent achieving level 5), but less well in maths (71 to 73 per cent). English results have yet to be published for England.
Test results in Wales were up on 2003 in all core subjects except Welsh, which stayed the same at 73.5 per cent.
A Welsh Assembly spokeswoman said the KS3 funding had not been stopped and continued to be distributed to LEAs via revenue grants. But officials are discussing whether the cash should be transferred to the Better Schools Fund (formerly GEST) from 2005.
LEAs decide whether the extra cash is set aside specifically for KS3 initiatives.
The spokeswoman added: "Any change in arrangements at a local level is a matter for the local authority, not the Assembly."
But John Hopkins, president of SHA Cymru, said the problem was an example of the "funding fog" in Wales. All the association's branch secretaries have been told by their LEAs that there is no direct funding for KS3 from 2005.
"The councils say 'We've not been given the money,'" he said.
"If the Assembly has moved money from specific grant areas into the general revenue grant without telling LEAs what it's done, we lose the money. It will probably get spent on filling holes in roads.
"This will lead to a cut in an area that the Assembly says is a priority. We will lose between pound;20,000 and pound;70,000 a year per school, used to maintain class sizes and promote initiatives."
St Julian's comprehensive in Newport now receives around pound;68,000 for KS3 work. It has ploughed cash into reducing class sizes, employing mentors and classroom assistants to support pupils, and buying new resources.
Head Steve Marshall said: "We have used the money to completely restructure our KS3 curriculum so we can more effectively target our teaching to the needs of pupils u and it has proved effective.
"We were looking forward to maintaining that structure, which is very expensive. If the cash is stripped out, we are going to be in a very serious situation."
Concerns about funding were raised this week by Assembly members at an education committee meeting about the budget.
Janet Ryder, Plaid Cymru education spokeswoman, told the committee: "Schools want to know that the money allocated to local authorities is actually going to the schools. But schools are telling us that the cash is not being translated into the school system."
Brushing aside the criticism, education minister Jane Davidson said: "It is up to the local authorities to determine priorities at local level."