Adequate funding to pay for teachers is "unlikely" to be available over the next few years, according to some primary and secondary heads.
However, Scottish headteachers were split, with half reporting they were "likely" to have the cash.
The heads were more united in their view that funds for classroom assistants were likely to be scarce - 60 per cent of both primary and secondary heads thought it was unlikely they would have adequate funds for assistants and support over the coming years.
The findings were revealed by the British Educational Suppliers Association, which contacted 512 schools via an online questionnaire about their spending on resources. It received 202 responses from 148 primaries and 54 secondaries.
Primary and secondary schools said they would be 7.2 per cent worse off in real terms next session compared with this year. However, secondaries felt they would be hit harder by the cuts, because pupil numbers were not falling as fast as in primary.
Investment in ICT hardware and software in Scottish schools was likely to suffer this year, the heads reported, with 27 per cent of primaries and 28 per cent of secondaries set to spend "much less" on ICT in 2010-11.
Nevertheless, Scottish schools planned to increase spending on books, the researchers found.
BESA claims per-pupil funding in Scottish schools is "significantly lower" than previously thought.
In March, the annual statistics for expenditure on school education in Scotland put the figure for primary education at pound;4,833 per pupil last year; in secondary, it was pound;6,665.
But this year, according to BESA's survey, primary heads predict they will have pound;2,900 to spend per pupil, and secondary heads pound;4,330.
The Scottish Government, however, warned that the survey only covered 8 per cent of Scottish schools and was lacking in detail about how it defined school budgets or per-pupil funding.
A spokesman said: "Not all expenditure on schools is undertaken directly by headteachers. Some is paid for centrally by the authority, and will not therefore have been counted in the BESA survey."