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.
Presently, the schools budget is £40bn a year.
Luke Sibieta, associate director at the IFS, said: ""The commitments made by each of the main parties would imply quite different paths for school spending in the next Parliament.
"Labour would increase spending per pupil by around 6% after inflation over the course of the Parliament, taking it to just above its previous historic high in 2015.
"Proposals from the Conservatives would lead to a near 3% real terms fall in spending per pupil over the Parliament, taking it back to its 2010 level."
The analysis of the headline figures comes as parents are preparing to hold events in different areas of England to highlight the cash crisis facing schools.
The IFS has previously said schools are currently facing the first real terms cut for 20 years.
Head teachers have been complaining of the tough decisions they have had to make about staffing and resources.

Subsea UK chief executive Neil Gordon said capital expenditure cuts and low crude price were starting to bite.
But he added that the fundamentals of the sector, which supports 60,000 jobs, remained relatively strong.
He was speaking at the start of Subsea Expo in Aberdeen.
Subsea firms play a key role in North Sea activities, which have been coming under pressure following a sustained fall in the price of oil.
This week accountants PwC said many oil and gas firms would need to transform the way they operate in order to grasp future opportunities in the sector.
Its report argued companies should be looking to deploy fresh strategies, including looking to reduce costs "in a sustainable manner".
Mr Gordon told delegates at Subsea Expo: "The UK's subsea sector came out of a strong 2013 to a relatively flat second half of 2014.
"The existing order book kept the industry going but, as this dries up and projects are abandoned or postponed until the oil price recovers, we are in for major challenges."
Mr Gordon said research suggested that overall capital expenditure growth forecasts should start to become more positive after 2016.
He continued: "It is therefore imperative that during this period Subsea UK demonstrates strong leadership by pushing the industry to more quickly adopt innovation and technology.
"We need to better explore how projects are currently delivered and then make the step-change to deliver major efficiencies.
"While subsea companies must speed up the development of new technologies, the industry must be prepared to embrace new technology.
"It has to become less risk-averse and more receptive to supporting field trials and implementation of new technology."
Subsea UK is urging companies to turn to the National Subsea Research Initiative, which brings academia and industry together to collaborate on getting new technology to market much more quickly.
NSRI will be focusing new research and development on a number of themes, including decommissioning methods and techniques, well intervention and small field cluster development.
A new Conservative government would leave schools in England worse off financially than they currently are, the Institute for Fiscal Studies says.
[NEXT_CONCEPT]
The UK's subsea sector is in for two years of tough times and will only weather the storm if it embraces innovation and new technology, an industry body has warned.