Primary schools in Lincolnshire are the worst-funded in the country, research for the NAHT reveals.
It shows that they received pound;2,297 per pupil this academic year compared to a primary in the London borough of Tower Hamlets which got pound;3,867 per pupil.
An average-sized Lincolnshire primary, with 200 pupils, receives pound;314,000 a year less than an average Tower Hamlets primary.
Helen Lockham, head of Bythams primary, in Lincolnshire, teaches two-and-a-half days each week, to reduce expenditure. She also covers for any absent staff.
"It's not ideal," she said. "I'm a great believer in small classes and one-to-one attention. But financially, it isn't possible. We've had to cut down the number of classes. And we're setting up a foundation-stage play area, but we can't afford water trays, sand trays or outdoor equipment. I feel like I'm depriving the children of something they're entitled to."
The NAHT research examined the per-pupil funding provided by councils.
Secondaries in Solihull were the worst-funded nationally, receiving Pounds 2,991 per pupil for the academic year 2005-6. But in the London borough of Kensington and Chelsea, an average-sized secondary was given pound;4,821 per pupil.
In a school of 1,000 pupils, this works out as a difference of pound;1.83 million.
Each authority receives a basic level of per-pupil funding from the Government, which is the same nationwide. But this is then supplemented with extra money in areas of deprivation, sparse population or high property prices.
Mick Brookes, NAHT general secretary, said: "The figures are shocking. The basic levels need to come up, to enable schools to do the job of education.
Low-level funding equals large class sizes. And it erodes leadership and management time.
"An enormous amount of money is coming in for city-based initiatives. But education in Middle England is just as important as in the inner-cities."
Bill Sedgwick, head of Alderbrook arts college, in Solihull, received almost pound;700 per pupil less this year than schools in Birmingham, only half a mile away. Yet the two authorities share the same catchment area.
He said: "Whenever any member of staff leaves, I'm always examining whether or not I need to replace them. There's always pressure to see if you can manage with what you've got, rather than bringing in new staff.
"Workforce reforms mean that we've had to bring in new support staff. But there's no real funding for it. Again, it puts an increased burden on the school's budget."