Schools in England are cutting back on teachers and teaching assistants (TAs) because of financial constraints, a survey for the Sutton Trust suggests.
The poll found 49% of primary schools and 57% of secondaries have cut back on TAs, while 18% of primaries and 54% of secondaries have cut teaching staff.
Information technology (IT) equipment and trips and outings are also being scaled back, it suggested.
The government said school funding was at record levels.
IT equipment is being cut back by 35% of primaries and 38% of secondaries, the survey of 1,361 teachers by the National Foundation for Educational Research found.
Outings and trips are being affected in nearly a third of schools - 30% of primary and 32% of secondary schools.
The poll also suggested some schools are using funding for disadvantaged pupils (the pupil premium) to subsidise the school budget.
Of the 420 senior leaders who took part in the survey, 32% of primary and 27% of secondary senior staff say the pupil premium is being used to plug gaps elsewhere in the school's budget.
Sir Peter Lampl, founder and chairman of the Sutton Trust and chairman of the Education Endowment Foundation, said: "Our new polling adds to the growing evidence from highly credible sources that the squeeze on school budgets is having a detrimental effect on schools.
"Many are having to get rid of teachers to close these funding gaps.
"Of particular concern is that schools are having to use funding for poorer pupils to plug gaps in their finances.
"The pupil premium should be used for highly cost-effective interventions such as peer tutoring and pupil feedback."
A spokeswoman for the Department for Education said: "All schools are trusted to use this [pupil] premium to ensure it meets the needs of their students and are held to account by Ofsted for how disadvantaged pupils benefit from the extra funding."
"We recognise that schools are facing cost pressures and we will continue to provide support to help them use their funding in cost-effective ways," the spokeswoman said.