Two secondary schools could take on 600 extra pupils between them after plans to build a new school became too expensive.
More places are needed as the Ipswich Garden Suburb development, which could comprise 3,500 new homes, is built.
But Suffolk County Council said building a new school in north-east Ipswich was likely to cost over £30m.
Northgate High School and Ormiston Endeavour Academy, previously Thurleston High School, are in negotiations with the authority about taking on more pupils.
Plans would involve both schools being expanded at a cost of about £20m, most of which would come from contributions from housing developers, with some council funding.
The new places could be available from 2027 but it is not yet known how many pupils either school would take.
Three new primary schools are still planned as part of the development.
The authority said, alongside the cost concerns, it believed falling birth rates across Suffolk and nationally meant there could be too few students attending in the future, potentially making a new school "financially unsustainable."
Andrew Reid, Conservative cabinet member for education and Send (special educational needs and disabilities) at the council, said: “With all new housing, it is essential that we match the number of school places with the predicted number of children and young people that may move in and to cover future need.
"This may mean we build new schools or explore the feasibility of expanding existing schools.
"In this case, expanding Northgate and Ormiston Endeavour seems to be the best option.
"Not only is it more economical, but both schools would benefit from the funding from additional pupils - and the new students would benefit from a wider range of opportunities.”