Edinburgh City Council has suggested children as young as 10 could complete their primary education in high schools in a bid to ease chronic overcrowding.
Education officers said there would be "benefits" in primary pupils being timetabled within the secondary school curriculum and having access to specialist teachers.
However, the idea has concerned some parents who said there would be risks in placing thousands of youngsters who were not ready into a secondary school environment.
Paul Godzik, the council's education convener leader, said: "The pressure on classroom spaces in our primary schools is one we are acutely aware of and, in the longer term, finding a sustainable solution to this problem is vitally important.
"We are always open to different solutions such as building new classrooms or creating extra space in existing schools. One possible solution put forward was relocating some P7 classes to secondary schools.
"We do not believe this is feasible in the short or even medium term, but the cross-party working group have requested we investigate if it could work."
However, Lindsay Law, a parent representative on the council's education committee, said the plan would cause unnecessary disruption as they have to get used to one new school, then another."
Acute accommodation pressure has resulted in some schools being forced to use dining rooms as gym halls, while many pupils regularly eat lunch at their classroom desks.