A major increase in demand for primary school places is set to put pressure on local authorities in the Anglia region over the next few years.
By 2015, Costessey, near Norwich, will have at least 75% more children than there are places currently available.
According to figures from the Local Government Association, by September 2016, Central Bedfordshire will need to find 25% more places while in Peterborough pupil numbers will increase by at least 20%.
Schools are already having to convert spaces such as music rooms and libraries into classrooms.
Others have been forced to reduce playground space or expand class sizes, the LGA warned.
David Simmonds, chairman of the LGA's children and young people board, said councils were facing "unprecedented pressures" on school places.