More than 4,000 headteachers have backed a letter to the Chancellor asking him not to cut school budgets.
Teachers from primary, special and secondary schools from 25 different counties have joined a campaign concerned about the Government's new National Funding Formula.
The formula is meant to make the allocation of school budgets fairer. But the Worth Less? campaign says it won't work.
The group is led by headteacher Jules White, whose own school, Tanbridge House in West Sussex, is one of the country's lowest funded.
Mr White admits his budget is so tight they've cut teachers and can't afford to properly use cleaners.
"Day to day it's desperate," he said.
"If I lose a teacher I can't afford to replace them. Our children most crucially are being disadvantaged as well. They're all going to sit the same GCSE yet they're in class sizes of 35, they don't get one-to-one support that they want.
"I'm really proud of the school, we're doing great, but think how well we could do if we were just funded adequately."
Even under the new formula Mr White reckons his school won't receive enough extra money.
He claims the Government's own figures highlight the imbalance.
Under the new formula, in 2018-19 an average funded primary school in Devon will receive £407,200 less than the same size school in the London borough of Greenwich.
An average secondary school in Oxford will get £4,050,200 less than the same size school in Hackney. That's the equivalent of 133 teachers.
We met year ten students during a maths lesson. They agreed the school's budget sums seem unfair.
One pupil said: "It's very unexpected, when you're at school you just think that you're the same as everyone else. We just feel a bit less worth. Like we're worth less than all those other schools."
Schools Minister Nick Gibb said: "The introduction of the National Funding Formula from 2018-19, backed by £1.3bn of additional investment, has been widely welcomed and will put an end to historic disparities in the system.
"There are no cuts in funding, every school will see an increase in funding through the formula from 2018."
But many dispute the notion of additional investment. Mr White believes in real terms, schools' budgets are still being cut by £1.7bn.
This is why on Tuesday representatives will deliver a letter to the Chancellor, Philip Hammond, ahead of this month's Budget.
Parents have been throwing their support behind the move too.
Dad David Gabriel said he's worried the next generation will suffer.
"As class sizes grow then my son's education and all the kids' here and in West Sussex are going to have a substandard education," he said.
"And he is actually, in the Government's eyes, worth less than children in other parts of the country."
Mr White says the support behind his campaign couldn't be stronger.
As he prepares to head to Number 11 Downing Street he said the message is simple: "All we're asking is he funds us fairly."Let's