Grange Primary School - just off Tower Bridge Road - was expanded in 2017 at a cost of £8 million as the child population in the borough was growing at the time.
Since then - with the combined effects of Brexit, the covid pandemic and economic strains - schools across Southwark and neighbouring inner London boroughs have seen a fall in the number of families applying for places.
If approved, the change in capacity from 60 pupils to a single class of 30 at Grange Primary would take effect from September 2025's round of admissions.
The proposed reduction in school capacity is part of Southwark Council's 'Keeping Education Strong' strategy adopted last year in response to a declining number of school-age children in the borough.
"This extensive work on falling school rolls exists because of a demographic crisis in London and beyond," Southwark's deputy leader Cllr Jasmine Ali wrote last June.
"We are not the only authority affected by this - we are the first to apply a comprehensive strategic approach.
"It is hoped this work will be instrumental in supporting our schools and communities through this difficult and challenging time.
"It will enable us to be ready in future years for further demographic shifts, in the event we see fewer or more pupils in Southwark."