Headteachers from 25 counties will be heading to Downing Street to appeal to the Chancellor to put more money into the education system.
A letter endorsed by 5,000 schools, including those in East and West Sussex, will be presented to Philip Hammond tomorrow (November 14).
Banding together under the WorthLess? campaign, the heads will tell Mr Hammond that the new National Funding Formula, introduced by secretary of state Justine Greening in September, had done "little to alleviate either the funding crisis and/or funding disparities that are engulfing schools across the country".
While the new system included investment of £1.3bn, the National Audit Office had already announced schools would face real-term cuts of £3bn by 2020, meaning they would still be £1.7bn worse off.
The headteachers will implore Mr Hammond to reinstate that £1.7bn into schools.
Their letter states: "Along with our parents and many other well placed stakeholders we are simply asking for the money that is being taken out of the system to be returned. Headteachers have played their part in running their schools to maximum efficiency.
"We have opened our books to the Department for Education and politicians. Nobody has suggested that money is currently being wasted.
"We have always recognised that there is not an endless supply of money but children and families deserve a fair deal and they are not receiving that at present."
Without the extra money, heads warned children's education would still be very much a "postcode lottery", with primary and secondary schools in one part of the country receiving hundreds of thousands and even millions of pounds less than similar schools elsewhere.