The amount of per pupil spending in England's schools has fallen by 8% since 2010, says an analysis by the Institute for Fiscal Studies.
The study says rising pupil numbers - and cuts to local authorities and sixth form funding - have seen a real-term reduction in school spending.
Heads say it disproves government "rhetoric" about record funding levels.
The Department for Education has promised an extra £1.3bn will be re-allocated to schools.
School leaders have been protesting that cash shortages have forced them to cut staff and to…