Four out of 10 of schools that took part in a survey said they were planning to shed staff.
A similar number said their budgets were falling, in the survey by the National Association of Head Teachers and the Times Educational Supplement.
The government in England says schools will receive an extra £3.6bn over the next four years.
Primary heads represented by the National Association of Head Teachers (NAHT) are meeting for their annual conference in Brighton this weekend.
About 1,300 heads and school business managers took part in the survey of the group's 28,000 members in England, Wales and Northern Ireland.New jobs
Four in 10 of those said their budgets were falling in the next 12 months, but the same number said their budgets were rising.
The remaining schools - two in 10 - said their funds would stay the same.
The survey suggests 37% of schools will cut staff, say those behind it.
The NAHT has extrapolated the figures from the survey to suggest 17,000 jobs will go and 5,000 will be created, leaving a net loss of 12,000 jobs.
But it says the figures for losses are conservative and that the situation will "get worse" during the next few years.
Cuts would involve teachers and support staff such as teaching assistants and administration staff.
The NAHT's general secretary, Russell Hobby, said: "Heads are working hard to keep their schools in the black and protect their workforce.
"We have not seen the full brunt of cuts yet, particularly as inflation is eroding the budgets."
Mr Hobby said it was clear from the survey there were "winners and losers" - with some school budgets rising while others were falling.
The government is directing more money at disadvantaged pupils through the new "pupil premium" - where money follows children from low-income homes.
But Mr Hobby said this was "compensating for cuts rather than adding new money to the system".
Although the main schools budget was protected in the autumn's Comprehensive Spending Review, ministers said that, with inflation, schools were facing a "flat cash settlement" and some would have less money.
Some schools are also feeling the impact of savings in other areas of spending by the Department for Education - such as money for particular schemes run in schools - and cuts in council spending.
Councils are facing a 27% cut over four years.'Fairer funding'
A spokesman for the Department for Education said the government was increasing investment in schools by £3.6bn over the next four years, protecting cash levels.
At the same time, it planned to improve the funding system to make it fairer and more transparent.
He said: "We're protecting the schools budget in cash terms per pupil, introducing a pupil premium for disadvantaged pupils, and putting money directly into heads' hands.
"School budgets fluctuate every year as pupil numbers change so it is normal for some schools to get more, and for others to get less. In fact this survey shows that around 40% of schools expect to see an increase in funding.
"With the introduction of the pupil premium, and a new fairer funding system, we will ensure that schools get the money they require to meet the needs of pupils."
Christine Blower, general secretary National Union of Teachers, said: "This is yet another example of how hollow the coalition government's words are about protecting schools' budgets.
"Many schools are already working with very limited resources. These cuts are a further devastating blow to education services and schools."
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