Spending on Scottish schools has decreased by around £540 per pupil since 2010-11, it has been claimed.
The new figures, published in The Times, were compiled by independent Scottish parliament researchers.
It has led to calls that school budgets, set to be announced by the SNP on Thursday, should be reversed and more funds put into education.
Statistics last week showed academic performance of 15-year-olds in Scotland has declined.
Opposition MSPs blame spending cuts and reduction of staff on the fall in standards.
But education secretary John Swinney has said he will drive through a “radical” programme of reform.
Now new analysis shows spending on schools has fallen from £6,692 in 2010-11 per pupil to £6,152 - a drop of more than eight per cent.
Kezia Dugdale, Scottish Labour leader, said: “On Thursday the SNP must not use the declining performance as political cover to justify centralising school budgets and support. That will not fix this. Nicola Sturgeon and her finance secretary Derek Mackay should halt these damaging plans and instead use the parliament’s powers to invest in our schools, ensuring they have enough teachers and support staff to reverse this decline.
The Scottish Government said: “Since the government took office, average spending per pupil has increased by at least 9 per cent in primary schools and at least 11 per cent in secondary schools.”