Schools in Rotherham and Barnsley are making cutbacks after revealing they do not expect to have enough Government funding by 2020.
John Healey, MP for Wentworth and Dearne, conducted a survey in his constituency in light of growing concern around the country about school funding.
Teaching unions say schools in the area face an average loss of nearly £400 per pupil under the Department for Education’s new funding formula, which aims to close the gap between different geographical areas.
Out of the 14 primary schools and two thirds of secondary schools that responded to Mr Healey's survey, 95 per cent think their funding from the DfE will be 'inadequate' by 2020.
All the schools who responded are already making cutbacks - 71 per cent have fewer teaching assistants, 71 per cent are not recruiting and 57 per cent are making staff redundant.
All the secondary schools said they were increasing class sizes and several primaries are considering it.
A total of 90 per cent of schools said their funding situation had worsened since 2010. The same amount said they had a budget surplus in the last three years, but 43 per cent predicted a deficit by 2020.
Mr Healey felt the findings were 'really worrying' and has written to the Education Secretary Justine Greening urging her to look at current funding levels and the proposed new National Funding Forumla, and give all schools the money they need.
He said: “We’re heading back to the class-size crisis of the mid-1990s. And I am appalled that schools in our area are having to cut staff.
“Headteachers should not have to make decisions like this. They should get the money they need to do their job and help our children get the excellent education and start in life that they deserve.
“Investment in our children’s education is investment in our country’s future.
“This government has the wrong priorities. Instead of funding all schools properly, they’re opening new free schools in areas which do not need them and spending £320m on selective grammar schools which will only help the favoured few.
“The government’s new funding formula simply redistributes an inadequate sum of money, and it will make matters worse for many of our local schools.”
A Department for Education spokesman said school funding was at its highest level on record, at more than £40bn in 2016-17.
He added: "But the system for distributing that funding across the country is unfair, opaque and outdated.
"We are going to end the historic post code lottery in school funding and under the proposed national schools funding formula, more than half of England’s schools will receive a cash boost. Significant protections have also been built into the formula so that no school will face a reduction of more than more than 1.5 per cent per pupil per year or three per cent per pupil overall.
"We recognise that schools are facing cost pressures, which is why we will continue to provide support to help them use their funding in cost effective ways, including improving the way they buy goods and services so they get the best possible value."