A new Conservative government would leave schools in England worse off financially than they currently are, the Institute for Fiscal Studies says.
The Tories have promised £4bn extra but this equates to a £1bn rise in real terms, the independent think tank says.
When growing pupil numbers are taken into account, investment per pupil will fall 2.8% by 2022, the IFS says.
Labour has pledged to increase spending per pupil by 6% over the course of the next Parliament.
The IFS says this would require an increase of around £4.8bn in real terms or 2017-18 prices.
The Liberal Democrats' plans protect per pupil spending over the course of the Parliament in real terms at the present level (for 2017-18).
This would require £2.2bn extra investment.