A new high school that opened its doors in February is already too small, forcing Edinburgh City Council to pay an extra £4.1m to extend the premises.
The £35m Boroughmuir High School will need to be upgraded to take in an additional 300 pupils by 2027.
Opposition councillors have accused the council administration of poor planning.
However, the authority said the rise in the number of Boroughmuir pupils could not have been predicted.
A number of other high schools in Edinburgh will also need to expand to accommodate more pupils over the next nine years, including Balerno, Currie, Trinity and James Gillespie's.
There are questions over whether there is adequate space at Boroughmuir to extend the building.
Alison Dickie, City of Edinburgh Council's vice-convener of children and families, said: "The school was originally built to accommodate projected future school rolls.
"Since then there has been a substantial increase in the number of pupils attending Boroughmuir from the feeder primary schools.
"This could not have been predicted when the size of the school was originally determined.
"We have therefore allocated £4.1m in our capital programme to build a new extension to address this issue."
The council will need to invest more than £91.5m by 2027 to upgrade high schools across the city to provide enough space for pupils.
Gavin Corbett, Green councillor whose ward covers Fountainbridge, said: "It's been obvious for quite a while now that the new Boroughmuir High School is going to be significantly over capacity. Indeed, it is already so.
"At present a feasibility study is being carried out to assess where best to provide the extra space that will be needed very soon. My own preference is that additional building is carried out west along Dundee Street."
"Could this have been foreseen? Almost certainly. The amount of student housing being built in the area has freed up former shared flats for families and hence school roll rises - that could have been factored in as part of the planning process."
Last month the council agreed to push ahead with plans for a mixed development at India Quay, next to the new Boroughmuir High School, including affordable housing.
Callum Laidlaw, Conservative education spokesman, said: "The new Boroughmuir High School is one of the most impressive in the city, but it's disappointing that the capacity issues were not better addressed by the SNP-Labour administration during its planning construction, given the projected growth in its catchment population.
"We are now in the unenviable position of trying to secure millions more for expansion, at a time where we already are struggling to fund repairs to our school estate. Failing that, the only alternative would be the uncertainty and stress of a catchment review. Hopefully this is a lesson to be learnt when we build our next round of schools."