Ministers are facing increasing pressure - including from a Wigan education group - to rethink school funding reforms.
Several borough head teachers and the Wigan Schools Forum have voiced concerns about the new funding formula in recent weeks.
But Theresa May has defended the controversial shake-up despite the threat of a back-bench rebellion within her party.
This week respected economic think tank The Institute for Fiscal Studies (IFS) warned some schools could face “protracted cuts” as a result of the shift from 152 different local authority formulae to a single national method of allocating funding.
It has been reported each of the borough’s secondary schools will see their budget cut next year along with 88 out of 99 primary schools.
The IFS analysis said the proposals were the most ambitious reform to the school funding system for over 25 years and, once implemented, would ensure similar schools in different parts of the country receive a similar level of funding per pupil.
But it warned that “implementing this reform at a time when there is already considerable pressure on school budgets will inevitably be difficult”.
The concept behind the reforms has been widely accepted but critics say it does not take into account rising costs.
The government has said it is investing more than ever before in the schools system but this has been rebuffed by union leaders who say there are more children in the system than ever before and the new formula will represent a real terms cut.
Chris Belfield, one of the authors of the IFS report, said: “Somewhat inevitably, this reform creates winners and losers, and it comes at a time of severe pressure on school budgets as we are currently in the tightest four-year period for per-pupil spending in English schools since at least the early 1980s.
“The Government has put in place transitional protections to help smooth the transition process up to 2019-20. However, there is significant uncertainty about what will happen after 2019-20.”
In a letter to parents, Wigan Schools Forum chair Adrian Hardy said: “It reads: “Fundamentally there is insufficient funding within the education system nationally and locally to allow schools to meet the budgetary pressures they now face.
“The proposals represent a lost opportunity to introduce a funding formula based on the actual costs of educating a child, regardless of where they live rather than be based on historical spend and use of national averages.”
At Prime Minister’s Questions, Mrs May acknowledged there would be a “number of views” but insisted “it’s not a vanity project to want every child in this country to have a good school place”.
Mrs May said the Government’s record had seen “protected school funding, more teachers in our schools, more teachers with first-class degrees in our schools, more children in good or outstanding schools”.