Disgruntled principals will descend on state parliament this week as the Australian Education Union blasts the Andrews government over school funding.
The looming showdown represents the first major headache with the teachers union - formerly a key ally - for the Andrews government, which pledged to make Victoria "the education state" ahead of the election.
The union's relationship with the Andrews government has soured following new laws that link private school funding to 25 per cent of that given to state schools.
The union is also calling on Labor to commit to the final two years of the needs-based Gonski school funding agreement, which was signed by the former state government and the Commonwealth in 2013.
AEU Victorian branch president Meredith Peace said the backlash from concerned principals - who will meet their local Labor MPs at parliament on Thursday- was "unprecedented."
"We have never had such a large group of principals saying I am prepared to get politically involved and make sure my local member understands how kids in their electorate are being affected by the decisions of this government, or the lack of decisions."
Ms Peace said schools wanted certainty about the fifth and sixth year of the Gonski agreement in next month's state budget.
She said Labor's controversial new legislation for non-government school funding was "disappointing" and would increase inequity in the state's education system.
"We don't believe it is either sector-blind or needs-based. We are very disappointed that the first bit of legislation that has come into the parliament goes against the concept of the Gonski model."
It is understood concerns about the new legislation have also been raised within Labor's own education policy committee.
But a spokesman for Education Minister James Merlino defended the legislation, saying it did not reduce the amount of funding available to state school students and was consistent with the Gonski reforms.
He said Victoria was "committed to the Gonski reforms" but did not explicitly say the state government would fund the final two years of the scheme.
"We will continue to take the fight up with Tony Abbott to make sure his government commits to years five and six of the agreement. The Andrews Labor Government is committed to supporting each and every Victorian child to receive the very best educational experience. "
Many principals have told The Age they have been forced to cut literacy, numeracy and welfare programs, as well as spending on maintenance, despite the promised Gonski funding.
An auditor-general's report released in February revealed for the first time that under the original six-year Gonski deal, Victorian schools would have received almost $4 billion from the Commonwealth in 2018 and 2019. Under the agreement, they were set to receive $3.2 billion in state funding over the same period.