School sixth forms were brought under the control of Wales's troubled post-16 funding agency this week - despite fierce criticism from headteachers and opposition politicians.
The National Association of Head Teachers Cymru says Education and Learning Wales has lost the trust of its members, while Plaid Cymru fears the agency will shut down sixth forms to save money - and that rural and Welsh-medium schools in particular will suffer.
A majority of one in a Welsh Assembly vote on school organisation regulations has given ELWa the power to open, close and reorganise sixth forms.
The vote came as it emerged that ELWa is paying pound;750,000 towards a three-year feasibility study on a proposed new lifelong learning centre in Merthyr Tydfil that could replace local schools' sixth forms.
Anna Brychan, director of NAHT Cymru, said it was concerned that ELWa had been given the power to close sixth forms.
She said: "We still don't know what criteria ELWa will apply to close a school sixth form. But its financial planning is designed to drive down costs to the level of colleges, contrary to funding promises made a few short years ago."
Gethin Lewis, general secretary of the National Union of Teachers Cyrmu, said it was not right to give ELWa "life or death" powers over school sixth forms when it had still to win the confidence of teachers.
Plaid Cymru has accused the Welsh Assembly government of sacrificing school sixth forms to ELWa. Its leader, Ieuan Wyn Jones, said: "Elwa will be able to alter sixth forms in religious schools which no local education authority can do.
"Sixth-form education - particularly in rural areas and in Welsh-medium schools - will not have sufficient provision to continue."
Janet Ryder, Plaid's shadow education minister, acknowledged the party had supported the creation of ELWa, but that "anyone looking at how it has developed knows it would not be wise to give it control of sixth forms".
But Jane Davidson, education and lifelong learning minister, told the Assembly that ELWa would work in harmony with local education authorities for the good of education in Wales.
"I will decide whether the case for opening, amending or discontinuing a sixth form has been made. Any current post-16 provision that is of high quality, relevant, value for money and fully supported locally does not face a threat to its future," said Ms Davidson.
In Merthyr Tydfil, a partnership including the LEA, Merthyr college and the University of Glamorgan has ambitious plans to create a lifelong learning centre covering post-16, higher, adult and work-based learning. Merthyr has one of the highest unemployment rates in Wales, yet employers struggle to find workers with relevant skills.
Vernon Morgan, the council's director of children's services, said: "The feasibility study will look at the best arrangements for lifelong learning and post-16 education for young people.
"This would open a range of vocational and academic opportunities for the whole community."
Grenville Jackson, ELWa's strategy and communications director, said any proposals arising from the study would be subject to full consultation, and the earliest date for changes would be 2007.
"Sixth form reorganisation is not a specific topic of the feasibility study," he said.
"The first two years will be spent exploring how we can increase participation and achievement levels. For example, at AS-level, there are probably about 15 to 20 subjects on offer in Merthyr.
"The study is looking at what it would take to double that number and increase the choice of learning available to individuals."
John Williams, head of 1,100-pupil Penydre high school, said schools did not feel threatened by the proposals. "Schools, parents and youngsters want to be sure that any new arrangements are at least as good as the existing ones, and preferably better. I think they will be. But there is still a lot to be worked out."
Alan Pritchard, head of Cyfarthfa high school, said Merthyr's schools were already working together in a consortium to broaden the range of AS and A-level courses available to their pupils. "We feel this is the way forward."
"We had a review of secondary education a couple of years ago and it was very apparent that parents, pupils and staff were in favour of retaining sixth forms."