The number of pupils attending England's secondary schools is set to rise by 20% over the course of the next decade, government figures show.
By 2024, nearly 3.3 million pupils are expected to be attending state-funded secondaries, compared with just over 2.7 million in 2015, a rise of 547,000.
The increase is mainly due to the upturn in the birth rate since 2002, the Department for Education says. The rise follows years of falling rolls due to low birth rates in the 1990s.
State primary schools in England will also see a rise in pupil numbers, although not as great as in secondary schools due to lower birth rates in 2013. The primary population is projected to be 4, 712,000 in 2024 - 336,000 higher than in 2015.
The DfE figures also show that projections in 2014 were slightly too low, with 75,000 more children than forecast set to enrol in state primary and secondary schools by 2024.
There will be 20,00 more pupils than originally predicted in secondary schools in 2024 and 55,000 more in primary schools by 2023.
But Russell Hobby, general secretary of the National Association of Head Teachers (NAHT), said some schools were "already stretched to their limits".
