Schools are facing the first real-terms cuts to their funding since the mid-1990s, a leading think tank has said.
Spending per pupil is set to fall by 6.5% by 2019/20, according to the Institute for Fiscal Studies (IFS), although it added that school funding has been well protected over the last two decades.
Instead it is sixth-formers who are facing a continuing squeeze on budgets, with spending per further education (FE) student falling by 6.7% between 2010/11 2015/16 and a further drop of 6.5% expected over the next few years.
It means that funding for 16-18-year-olds is no higher than it was almost 30 years ago.
The IFS study examines education spending for different age groups - from early years to universities - over a number of years.
It shows that the biggest spending increases over the last 20 years have been on schoolchildren in England, with £4,900 currently spent on each primary school pupil £6,300 spent per secondary student. In both cases this is around double, in real terms, the amount spent in the mid-1990s.
But the report shows that school spending is now falling, and will drop by 6.5% over the course of this Parliament.
"This will be the first time schools have seen real-terms cuts in spending per pupil since the mid-1990s", it says.
The IFS also says that protections for school budgets over the last parliament mean that spending per pupil will still be similar to 2010 levels,
And it notes that the introduction of the national funding formula, which will redistribute money to schools in a way that ministers say will be fairer, is the "largest shake-up in school funding in England for at least 25 years".
The IFS report warns that 16-18 education has been "the biggest loser from education spending changes over the last 25 years".
"It experienced larger cuts in the 1990s than other sectors, smaller increases during the 2000s and is currently experiencing the largest cuts," it says. "This long-term squeeze in resources is a major challenge for the sector as a whole."
FE spending per student was 45% higher than secondary school spending in 1990, and will be around 10% lower in 2019/20.
Luke Sibieta, one of the report authors and an IFS associate director, said: "Over the next few years, both further education and schools are due to experience cuts. For schools, this comes on the back of very significant increases over the last few decades.
"For FE, this comes on the back of tight funding settlements for decades that will leave spending per student the same in 2020 as it was in 1990. The lack of priority given to FE by successive governments in spending settlements does not seem sustainable."
A Department for Education spokesman said: "School funding is now at its highest level on record at more than £40 billion in 2016-17 and the IFS has shown that by 2020 per pupil spending in schools is set to be at least 70% higher in real terms than it was in 1990.
"We are transforming post-16 education and investing £7 billion to ensure there is a place in education or training for every 16 to 19-year-old who wants one."
Liberal Democrat leader Tim Farron said: "These cuts could be the death of education as we know it.
"Smaller budgets will mean staff redundancies, bigger class sizes and slashing things like the school clothing grants that help some of the poorest pupils.
"These economies could also mean the end of the teaching assistants in the classroom.
"The Tories have no shame. Tax cuts for big business while they cut the education budget."
Shadow education secretary Angela Rayner said: "The Tories are breaking their promise to protect per-pupil spending, and it will be children all across the country who pay the price for their failure.
"Their cuts to further education especially are leaving behind thousands of young people that want to pursue technical skills and education and once again (they are) proving themselves to be the party of the few, not the many."