Up to 700 teaching and other school jobs could be lost in Cardiff over the next five years because of falling rolls, headteachers fear.
The figure emerged as Wrexham became the latest Welsh council to review surplus places. It will have 3,000 empty primary desks by 2009.
But government initiatives and retirements from Wales's ageing education workforce could mean more, rather than fewer, teachers will be needed in future.
Lower birthrates across Wales mean many schools have more spare places.
Pupil numbers are predicted to fall by 45,700 between 2003 and 2013 - a drop that could, in theory, cost the jobs of around 2,500 teachers.
But more than 9,169 teachers over 50 are registered with the General Teaching Council for Wales, and would be expected to retire in the next 10 years. Last year, 280 retired at 60 and another 370 went prematurely - more than enough to cover falling pupil rolls.
A review group set up by the Assembly government is investigating how many teachers Wales needs, and is to report in the autumn. Geraint Davies, secretary of teachers' union NASUWT Cymru, submitted the GTCW figures in his evidence to the group.
"We can't come to the simple conclusion that if there are x thousand surplus places, y thousand teachers' jobs have to go. A few years down the line, we are going to hit a massive crisis because of the ageing profile of teachers in Wales," he said.
Anna Brychan, director of the National Association of Head Teachers Cymru, is concerned that school reorganisations aimed at reducing surplus places will give teachers a "gloomy picture of their future job prospects".
"Huge numbers of teachers are due to retire over the next few years and the last thing we want is a climate which causes teachers to drift away from the profession."
However, in the short term, many teachers face uncertain futures as LEAs grapple with surplus places. In 2003, there were 46,013 empty desks (15.6 per cent) in Welsh primaries and 22,206 (9.3 per cent) in secondaries.
In Cardiff, more than a fifth of primary (6,466) and 7.6 per cent of secondary places (1,761) are empty, costing an estimated pound;2.75 million a year to maintain. Total pupil numbers are expected to fall by a further 6,549 by 2013.
The council has completed an initial consultation on how it will reorganise the capital's schools, and will spell out in the autumn which ones will be affected.
Heads have written to chief schools' officer Hugh Knight, urging the council to act. "The management of a possible loss of 700 educational posts in the city needs to be addressed now," they say.
Geraint Rees, head of Ysgol Plasmawr and chairman of Cardiff's secondary headteachers' conference, said the council had not involved school leaders, despite offers of help.
"We know change has to happen, but we and our colleagues would like to be informed," he said.
The heads' letter also raises concerns about how improvements to surviving schools will be funded, warns against selling off all redundant sites for housing, and challenges the council's recommended ideal sizes for primary and secondary schools.
A Cardiff council spokesperson said it was not yet possible to estimate job losses. It has pledged to redeploy staff and seek voluntary redundancies, but some staff may be made redundant.