The number of secondary school pupils in England is set to increase by 20% within the next 10 years, dramatically reversing a decades-long decline.
Some 3.3 million children are expected to be attending state-funded secondary schools by 2024 - the highest total for over 20 years.
The overall number of pupils in state-funded schools in England is also projected to rise, jumping by 13% to about 8.2 million.
The boom in the secondary school population is due mainly to the increase in the UK birth rate since 2002, according to the Department for Education (DfE).
Primary schools are not expected to see a similarly steep rise, however.
The number of pupils in state-funded primaries is projected to increase by just 8% by 2024. This is because the UK birth rate is expected to slow over the rest of this decade.
The size of the primary school population is also forecast to show a slight fall in 2024, which would be the first year-on-year drop since 2009.
Secondary pupil numbers have been declining in England as a result of low birth rates in the 1990s, and are projected to be 2.7 million in 2015, a fall of 9% since 2004.
But an increase in the birth rate that began in 2002 is about to start feeding into the secondary school population, triggering what the DfE anticipates could be a rise in numbers of a fifth within 10 years .
By contrast, the primary school population is about to be affected by a decrease in the birth rate that began in 2013.
Forecasts for independent schools show numbers remaining broadly static over the next four years.
A DfE spokesperson said:
"We want every parent to have access of a good school place for their child. Despite rising pupil numbers, 95% of parents received an offer at one of their three preferred schools.
"But we recognise that as the population grows the demand for new school places increases. That is why we doubled the funding for school places to £5 billion in the last parliament, which has helped create almost 500,000 new school places. A further £7 billion has already been committed to create even more places over the next six years.
"With a million more pupils in good or outstanding schools compared to 2010 it is clear that schools and councils are responding well to the challenge of increasing pupil numbers."