Eleven primary and six secondary schools could close in Cardiff as part of a wholesale reorganisation.
The plan, which is being put out to consultation, has been drawn up because the city has 8,000 spare places.
BBC Wales has obtained a leaked copy of the proposals which also outline plans for three new secondary schools and the expansion of some Welsh-medium schools.
The timescale of closures varies with some years away. Cardiff Council said £300m is promised for school repairs.
The council says like many local authorities in Wales it has been facing the challenge of surplus school places and falling numbers and it currently has 8,000 spare in Cardiff, costing £3m a year.
It has been instructed by the Welsh Assembly Government and the inspection body Estyn to reduce them.
Llanedeyrn, Llanrumney, Rumney, Cantonian, Mary Immaculate and St Illtyd secondary schools will all close under the proposed shake-up.
Three new secondary schools - one a Welsh-medium - will open. A Roman Catholic secondary will replace the lower school at Whitchurch High, currently the city's largest school, and a community secondary will open in Rumney.
Primary schools across the city are listed for possible closure. However, two primaries in Pontprennau, which currently has no schools, and another Welsh-medium institution in the Trowbridge area will also be built.
Some of the closures listed in the consultation document are up to seven years away but the school serving Pontprennau and Old St Mellons is scheduled to open in September 2008.
Changes to catchments areas are also included in the proposals as well as a commitment for £300m over a period of 10 to 15 years to deal with the repair backlog at school buildings.
A council spokeswoman said:"It is a net reduction of 15 schools - not counting amalgamations and not counting nurseries being incorporated into adjacent primaries".
Cardiff council leader Rodney Berman defended the reorganisation plan saying that they had to face up to the problem of over-capacity.
"The aim of these proposals is to ensure that the best possible education is available to all children in Cardiff for generations to come."
He said the "unprecedented" £300m investment plan was the "foundation stone" for achieving the council's vision for a "high quality, innovative and inclusive education system".
"We are aiming to develop the highest possible quality of education in Cardiff's schools so that all pupils can achieve high standards."
Mr Berman added that the plan would be open to consultation with parents and other groups and he was looking for responses before any final decisions were made.
Assembly education minister Jane Davidson has refused to comment on the plans which were outlined to head teachers in the city earlier on Wednesday.
The proposals are being made public on Thursday at the start of a three-month consultation period.
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