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The Liberal Democrats have unveiled their pledge on education spending, promising to trump both Labour and the Conservatives.

Education budgets are under pressure from rising prices and pupil numbers, which could leave schools worse off unless funding increases.

Remember we are only talking about England here - education policy is devolved to Scotland, Wales and Northern Ireland.

Labour promised to ring-fence the entire education budget, from pre-school up to the age of 19. This means the amount of money available each year will stay the same and it won't be eroded by inflation. However, the number of schoolchildren is expected to increase by about 460,000 between now and 2020. With this growth in the population the amount of money spent on each pupil will fall.

The Conservatives have looked at the funding question differently. They say that they will protect the schools budget for children aged five to 16 years by protecting funding per pupil. In other words, they will keep the amount spent on each pupil the same, so school funding will rise in line with pupil numbers, but it will not necessarily keep pace with inflation.

This is a change of policy from the Tories, who over the past five years have protected schools against inflation, but not rising pupil numbers. They have not committed to protect the pre-school and further education budgets.

The offer from the Liberal Democrats is the most generous of the three. Not only will they ring-fence spending "from cradle to college", but later in the Parliament they will also inject extra money into the system to effectively increase funding in line with both pupil numbers and inflation.

Bad forecasters

They say that once they have balanced the books by 2017-18, they would then increase funding in line with economic growth (that's inflation plus the rate of growth in the economy). This would mean a sharp increase in funding from 2018 onwards, which would help schools account for the rise in pupil numbers. So even with an increase in the number of school-age children, the money spent on each child would have risen in line with inflation by the end of the Parliament.

The Liberal Democrats say that this is equivalent to an extra £2.5bn on top of what Labour is promising, and £5bn more than the Conservatives, but it does rely on them managing to sort out the deficit by 2017-18 and the economy growing. Over the past five years, the government, of which they have been part, has been very bad at predicting when the deficit will be eliminated.

The Liberal Democrats say they will spend so much more than the Conservatives because the Tories are only protecting spending on education for five to 16-year-olds. The £5bn figure is based on Institute for Fiscal Studies analysis, which assumes that the Conservatives would cut spending on pre-school and education for 16 to 19-year-olds in line with other unprotected areas.

Whether you protect for rising prices or rising pupil numbers is particularly significant at a time when pupil numbers are rising. A from the National Audit Office found that there was a strain on school places because between 2001 and 2011 there had been the largest 10-year increase in children born in England since the 1950s, with live births rising 22% to 688,000.

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