More details about the future of high schools in Powys have been outlined in a new council document.
Seven models, including options for all-through schools - where pupils remain throughout their education - and possible closures, have been drawn up.
A shortlist of preferred options is expected to be revealed in September following a consultation process.
Four options from an preliminary council report were leaked to the media in January.
It revealed that several schools were under threat.
The new document has fleshed out the council's plans with seven models drawn up for the future of schools and three options for post-16 education.
This includes a proposal to scrap sixth forms and hand responsibility for post-16 education to the county's further education college, Coleg Powys.
The document also reveals that models might be set up for different areas of Powys, although one option is to "do nothing" and continue with the current schools set up.
Councillor David Jones, who is responsible for schools, said: "Secondary schools in Powys are facing their greatest ever challenge.
"Our schools have delivered high quality services for generations, but it becoming clear that they are facing serious financial decline and are in danger of becoming unsustainable.
"Falling pupil numbers, changes in education and legislative requirements and a need to provide sustainable Welsh medium provision and transform post-16 learning in the county is putting our schools in a perilous position.
"Pupil numbers have been falling for the past five years and are predicted to continue that trend to 2015. As a result 15% of secondary school places are surplus at a cost of £1m per annum and that percentage will increase to 25% by 2015, doubling the annual cost."
Mr Jones said a number of schools were "already experiencing severe budget deficits and are being forced to take very difficult decisions as a result".
He added: "In publishing this strategic document our vision is simple. We want to produce the highest quality bilingual provision with a broad range of learning opportunities, supported by world-class facilities and technology that will enable all Powys learners to develop their skills to the highest level."
Plans to modernise high schools in Powys have provoked strong objections from parents and pupils.
There have been a number of public meetings, a petition and campaigns set up on Facebook.
Montgomeryshire MP Glyn Davies said: "Since the county council's plans to reorganise secondary education in Powys was made public earlier this year, I have supported an option based on 'federation'.
"It is just not acceptable to contemplate closing any of our secondary schools, or removing their sixth forms. I accept that the 'status quo' is not really an option, and that cost reductions will have to be made.
"The only way to achieve this is by much closer cooperation between schools."
Montgomeryshire AM Mick Bates said: "Powys County Council is moving forward in a positive way, inviting consultation which I believe is crucial at every stage of this process, to ensure the best solutions for each community are identified."