The BBC with half an explanation

This BBC report starts thus:

'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.'

Well partially yes. But surely the part missing from the explanation is the phenomenonal increase in immigration post 1997. How many of the rise in secondary school pupils is accounted for by the children of people who immigrated to this country post 1997? Immigrants who had children as late as 2002 would now see those children entering secondary school this September. What was the rise in immigration between 1997 and 2002?