School across Wales have nearly 76,000 surplus places, the BBC has found.
The survey comes as Cardiff Council announced 11 primaries and six secondaries could close because the city has 8,000 more places than needed.
Pupils at Cantonian High in Fairwater, which is on the closure list, staged a demonstration on Thursday.
The Cardiff proposals, which are going out to consultation, also outline plans for three new secondary schools and the expansion of some Welsh-medium schools.
Councils across Wales say they face similar problems to the capital city.
The total number of surplus places stands at 75,822.
Rhondda Cynon Taf has more than 10,000, Swansea nearly 5,000 and Wrexham over 4,000.
Details of Cardiff's proposals were leaked on Wednesday before the council officially published them on Thursday.
Chris Howard, National Association of Head Teachers (NAHT) spokesman for south Wales, agreed there was a problem in Cardiff and across Wales with surplus places.
"Everybody in education concedes that," said Mr Howard.
"The eyes of Wales will be on the capital city from this point on to see the planners in Cardiff get the decision-making right.
"[They have to] get the process right and look after the interests of students, children, parents and their teachers and head teachers," he told BBC Radio Wales.
Potentially, more than 300 schools in Wales could close if all the surplus places were to be removed, although Welsh-medium and faith-based schooling as well as Wales' geography means the matter is more complex than simply numbers.
Steve Reardon of teaching union NASUWT warned unions would fight any compulsory redundancies if schools did close.
"In these schools that have been earmarked, there will be some blight and there will be a problem with recruitment," he said.
"As far as the NASUWT is concerned, we will look after the interests of our members and we will take a full part in the consultative process.
"As a trade union, we will not be able to accept compulsory redundancies. We believe that it's a manageable process over the period the council has in mind and we will fight for teachers to keep their jobs."
In Cardiff, Llanedeyrn, Llanrumney, Rumney, Cantonian, Mary Immaculate and St Illtyd secondary schools could all close under the proposed shake-up.
Three new secondary schools - one a Welsh-medium - will open. A Roman Catholic secondary could replace the lower school at Whitchurch High - currently the city's largest school - and a community secondary may 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, and £300m is pledged to carry out a backlog of repairs.
What is your reaction to the proposed closure of schools? Does your local school face being closed or merged? Send us your comments or experiences using the link below.