Headteachers have warned a freeze in education funding could plunge secondary schools into crisis, with almost all expected to run out of money and reserves.

School leaders and politicians predict that freezing the money allocated to schools will amount to an effective cut of £23 million, with the equivalent of a £60 million cut by 2020.

Heads say they have already had to stop replacing staff who have left, and have had to removed subjects from the curriculum.

Opposition politicians warn of larger classes, sixth form closures and fewer teachers and teaching assistants per pupil as schools try to balance their budgets.

Ed Conway, headteacher of St Michael Catholic High School in Garston, told the Observer that he did not replace seven members of staff who left this year, and has had to cut music, Spanish and dance from the A-level curriculum, and that He says redundancies are “inevitable”.

He said: “We are in uncharted territory and this is an unprecedented problem.

“Schools with 1,000 or more students are getting hundreds of thousands of pounds less money each year.

“We had to restructure last year, and the larger class sizes are affecting vulnerable children, who are not getting the same support as they did because we cannot afford it.

“We are trying our best to protect the curriculum, but have already had to make changes and will probably have to again.”

Mr Conway said cuts would have a knock-on effect and any money reserved to repair school buildings would have to be spent on other things.

He added: “It is a very challenging time for all schools in the county, we are all facing the same pressures. We are now having to give more work to a smaller work force.

“There is talk that some schools could be half a million pounds in debt in the next few years.”

Headteacher of Watford Grammar School headteacher Dame Helen Hyde DBE said the funding reduction is cuts are already having a negative impact in schools and will only worsen.

She said: “Obviously it is a real concern. We have outstanding schools that are well run and efficiently managed.

"We've already squeezed our budgets to the limit to such an extent that the next step will impact on children’s learning. Our only alternative is to reduce staff and increase class sizes.

“We have no choice. An essential service is being cut. Our future is trying to maintain the highest possible standards and care for our students, which requires experienced staff. What are we doing to help the most important assets of our country?”

Liberal Democrat county councillor Mark Watkin, opposition spokesman for education, said: “Secondary schools are facing a major situation.

They haven’t got the funds to remain viable unless they make some cuts.

“There will inevitably have to be major changes in the way schools operate and almost certainly there will be larger classes, fewer teachers and fewer teaching assistants as schools try to balance their budgets. Some schools will have to close their sixth forms.”

Cllr Watkin last week told the council's education committee that the heads of Bushey Meads, Watford Girls Grammar, Parmiter's, Queens, Rickmansworth and St Clement Danes schools had written to their MPs in March warning they expected a deficit of £200,000 to £300,000 this year.

He said the crisis has been ‘coming for some time’ due to a reduction in sixth form funding, which had previously allowed schools to use the money in other areas.

Alan Gray, chairman of the Hertfordshire Schools’ Forum – made up mostly of headteachers – has written to education minister Nicky Morgan on a number of occasions expressing grave concern about the funding crisis.

Mr Gray said headteachers were waiting for the new spending review to be announced to see if there is any change to original plans.

The county council’s education committee last week defeated a motion for the local authority’s leader, Robert Gordon, to follow suit.

Senior education officers had warned Hertfordshire County Council’s education committee of the impact of the freeze.

Cllr David Williams, cabinet member for education at County Hall, said: “A number of headteachers are concerned about the national funding settlement for schools.

“It does need to be recognised that schools are relatively well protected compared with many other areas of government.

"Following the general election, the government established protections for schools’ spending, including a commitment to protect spending per pupil in cash terms for the life of the parliament.

“Our focus is to ensure there are the tools and measures in place to enable schools to manage cost pressures and support for the key area of teacher recruitment and retention.”