The number of pupils in England's secondary schools is set to rise by almost a fifth over the next decade.
Government figures show there is expected to be around half a million more secondary-age children by 2026.
The increase is being fuelled by a rising birth rate in the early 2000s which has led to growing numbers of pupils making their way through the school system.
The latest government projections show that overall, pupil numbers are expected to increase by 8.7% between this year and 2026, rising by around 654,000 to around 8.1 million.
In secondary schools alone, the overall population is projected to reach around 3.3 million in 2026, a 19.1% increase (around 534,000 more pupils).
This is the second year running that secondary school numbers have risen, the Government report said.
It added: "This is primarily because increased births from 2002 onwards means there are now larger numbers entering secondary schools at age 11 than are leaving them at age 16."
Primary school numbers are projected to be around 100,000 higher in 2026.
"Direct immigration of pupils born outside the UK has a very small effect on the school age population," the Department for Education report says.
"However, the birth rate, which has a much larger effect, is in turn affected by any increase in the number of children born to non-UK born women (who overall tend to have higher fertility rates)."