A one-off increase of £3.8bn would be needed to reverse 8% cuts in per pupil school spending, new analysis shows.
The Institute for Fiscal Studies (IFS) says a further £1.1bn would be needed each year up until 2023 to maintain spending in real terms, once rising costs were taken into account. Candidates for the Conservative leadership have been making competing promises on education spending.
The government says funding for schools has been at its highest level ever. School budgets have moved up the political agenda, partly as a result of campaigning by parents and head teachers across England.
IFS analysis says since 2009, spending has fallen by 8% per pupil once rising costs such as pay and pension contributions are taken into account.
In a new analysis published on Tuesday, the IFS says to reverse that real terms fall for 5 to 16-year-olds would take a one-off increase of £3.8bn. That includes all spending in schools, including that by local authorities, and is more than any candidate has promised so far.
To keep up with rising pupil numbers, the IFS says £1.1bn a year would be needed to avoid future real terms cuts. A similar 8% increase for 16 to 19-year-olds would cost around £480m as a one-off uplift. Sixth form and further education colleges have faced the sharpest squeeze in budgets in recent years.
But with some Conservative leadership contenders also suggesting tax cuts, it's not clear where the money would be found.
Mr Sibieta says making education a priority would deepen the squeeze elsewhere, unless the government increased borrowing.
There is also a subtle rebuke for Boris Johnson from the IFS, after he described variation in funding per pupil in different parts of England as a "postcode lottery".
A new funding formula for schools is being gradually introduced, based a combination of measures of need in different local authorities.
However, many in the lowest funded areas, which include many conservative heartlands in the counties and shires, remain disappointed that the formula will not lead to substantial increases for all. The government has argued that school spending in England is at its highest level ever in cash terms.