The council said discussions were at an early stage
The future of several secondary schools is under threat after a council revealed it was carrying out a review.
Powys Council said it was prompted by falling pupil numbers and "severe" financial pressure on schools although it was "far too early" for a decision.
It is understood four options have been drawn up, but this could result in up to seven secondary schools closing.
They could be replaced with "learning centres" and "centres of excellence" providing vocational courses.
The NASUWT teaching union warned of industrial action if there were compulsory redundancies, while some politicians criticised the council's plan.
Powys council is reviewing the provision of primary and secondary schools.
By Colette Hume, BBC Wales education correspondent
This review spells uncertainty for the county's secondary schools. Whatever happens, the results will radically change the face of secondary education for thousands of pupils. Money is one of the biggest problems: the number of children is falling and the cost of keeping classrooms open is rising. Powys says severe financial pressures mean change is needed.
Powys isn't the only county considering massive changes to education. Merthyr Tydfil plans to close its sixth forms and centralise post 16 education. Powys is also considering that among other options.
Any changes will face strong opposition from parents, unions and politicians all keen to preserve their schools. Some of those facing closure provide some of Wales best exam results. Their - in some cases - very small sixth forms regularly sending students to Oxbridge.
Powys says no firm decisions have been made on the future of the county's secondary schools. But according to a leaked document if the plans get the go-ahead changes could begin to take place as early as next year.
It is also facing a budget deficit of £26m over the next five years and confesses it is facing some tough decisions.
Officials are finalising plans to close 10 primary schools in an area of south Powys and build four new replacements at a cost of £20m.
Powys council chief executive Jeremy Patterson said: "While the council has embarked on a review of secondary education and produced initial models for discussion no decisions have been made - it is far too early for that.
"The fact that the council is looking at secondary schools should not be a surprise, it would be more of a surprise if it was not looking at the future structure of the sector.
"Powys like other councils is witnessing falling pupil numbers and schools are facing significant budget deficits.
"The county must modernise its secondary sector if it is to provide the best possible education for its youngsters. Change is not only inevitable but essential to provide a world class service for future generations."
He said that the plan required "considerable debate within the council and wider communities before any decisions are taken".
The NASUWT, the teachers' union, said jobs could be at risk.
Seven schools being considered for closure as one of the options by Powys Council
Spokesman Rex Phillips said: "Unless the Powys authority can give a guarantee of no compulsory redundancy, the threat of whole-scale school closure inherent in these proposals could lead to industrial action across the authority."
NUT Wales secretary David Evans said the options had been presented before "meaningful" discussions with those who would be involved.
"The financial pressures imposed upon secondary schools in Powys have been a significant issue for several years," he said.
"Whilst the NUT would accept that a review of provision is necessary we would oppose any plans to replacing secondary schools with so called 'learning centres' or centres of excellence'".
It is understood the council has drawn up four options:
One suggestion is to create sixth form centres at Coleg Powys, a further education college in Newtown, Llandrindod Wells and Brecon, although five schools would shut as a result of this.
- Another option favours keeping Brecon and Newtown high schools open, while two bilingual "learning centres" would be created in Welshpool and Llandrindod Wells. This would also involve seven schools closing.
- A third option would involve two centres of excellence at Brecon and Newtown, with up to four schools closing.
- Another option would involve Brecon and Newtown centres of excellence and five learning centres on eight sites.
Brecon and Radnorshire AM Kirsty Williams described the plan as "hugely controversial".
"If what has been leaked is correct, I think they are hugely controversial and I say that not just as a politician, but as a parent of three children who will be directly affected by these proposals," said Ms Williams, who is also leader of the Welsh Liberal Democrats.
"I think the council, if they persist, on the basis of these leaked reports, I think they will have a fight on their hands."
Nerys Evans, Plaid Cymru AM for Mid and West Wales, said the council should "prioritise" funding for education.
She added: "This news will be extremely worrying and I hope Powys council consults fully on all options available to it.
"As Powys council has highlighted, councils throughout Wales are facing serious funding shortages due to the cuts to the Welsh budget by the Labour government in London.
"But council should clearly prioritise essential services like education with the funding they have. The One Wales government has already announced a review of spending on education and I fully support that."
Labour Mid and West Wales AM Alun Davies said the council's proposal was "shocking".
He added: "The Powys Lib Dem/Independents coalition must have taken leave of their senses.
"At a time when the new Labour first minister is launching a review of education expenditure we have Lib Dems in the council closing schools and Lib Dems in the Assembly refusing to support the review.
"Parents, teachers and schoolchildren in Powys deserve far more than poorly though out documents presenting us with a fete accompli."
Conservative prospective parliamentary candidate in Montgomeryshire, Glyn Davies, said: "We have known for some time that Powys County Council's financial situation will not sustain our secondary education system as it stands.
"However, I do not accept that closing the high schools at Llanidloes, Llanfyllin and Llanfair Caereinion is a remotely acceptable option."
Mr Davies said closing high schools in Llanidloes, Llanfyllin and Llanfair Caereinion would have a "devastating impact" on the entire social structure of Montgomeryshire.