A number of schools in Cardiff are set to close under plans to cut 8,000 surplus places in the education system.
New schools could also be created in some areas, the council say
The council, which said the places cost taxpayers £3m a year, will unveil draft proposals on Thursday.
Council leader Rodney Berman admitted closures and mergers would happen, but denied a figure of 18 claimed by opposition parties, who say the process has been too secretive.
He said the plans would go out to a full three month public consultation.
The plans are also likely to create extra Welsh-language school places.
As yet, there are no definite figures but some educationalists and politicians estimate up to 18 of the city's 142 schools could close and sites could be merged.
According to Mal Davies, National Union of Teachers spokesman and head teacher at Willows High School in the Splott area of the city, the extra places cost the community as much as if they were occupied by a pupil.
"The option of doing nothing is really not there and we are damaging the education of the youngsters in school because of the places we are paying for of the youngsters who are not in school," he said.
Rodney Berman told the BBC Wales' Politics Show that the proposals were not just about schools closing.
"There will be schools created because there are some parts of the city where we have undercapacity, communities like Pontprennau which don't have a school at all," he said.
"There is also a lack of capacity in the Welsh-medium sector so we'll have to create to create new Welsh-medium schools.
"There will be some school closures, I'm not going to pretend otherwise, but I'd like to see this as an opportunity to restructure the schools system in Cardiff."
He added the council was under instruction from the Welsh assembly government and inspection body Estyn to reduce the number of places in Cardiff.
The draft proposals would be out to consultation later this week until July, giving all parents a chance to attend public meetings.
'Heart of community'
Labour group education spokeswoman Georgina Phillips feels there has been a lack of consultation in the process.
Parent Jonathan Atkinson says there is panic through lack of news
"There has been a tremendous amount of concern amongst the parents and the children and the teachers who have absolutely no idea at all what's going on and I think that has been totally unacceptable," she told BBC Wales.
Parent Jonathan Atkinson agreed many people were worried by the lack of information so far.
He said no-one knew whether Llanrumney High School - like any other - was in danger.
"I think that parents need to know, they need to plan ahead. And they've left it so late people are just gone into a panic."
He called the school "the hub of the community",
"If you take the school away you'll take the heart out of the community and that'll be the beginning of the end I think - and so do other people".
Head teachers and councillors like Plaid Cymru's group education spokesman Delme Bowen will find out more about the plans a day ahead of the public on Wednesday.
He does not want to see 18 schools lost to the public.
"We think that many schools should be used as a social asset, a community asset," he said.
"We don't think that they should rush to close schools until they fully explore the community impact, or what the community asset of that school is. And that hasn't been done as yet."