"Under your plan, 87 per cent of public schools will stay below the minimum level of funding required to meet the educational needs of their students by 2023," the letter says.
"While private schools, the vast majority of whom will be above that benchmark, will receive a special deal of $4.6 billion."
The letter also urges the Morrison Government to fund all schools to 100 per cent of the agreed schooling resource standard, which is the basis of the needs-based funding model, by 2023.
That would upgrade facilities through a capital works kitty for public schools and reverse funding cuts for students with disability.
The Federal Government is tipping an extra $23.5 billion over 10 years to all schools from next year, which is $17 billion less than Labor had promised under the Gillard Government's original plan.
"This impacts most severely on Aboriginal and Torres Strait Islander children, children with disability and children from disadvantaged backgrounds," the letter reads.
The letter's 26 signatories include:
- Australian Education Union
- Australian Secondary Principals’ Association
- Australian Special Education Principals’ Association
- Australian Government Primary Principal Association
- Children and Young People with Disability
- SNAICC – National Voice for Our Children ACT Principals Association
- NSW Secondary Principals' Council (NSWSPC)
- NSW Primary Principals’ Association (NSWPPA)
- Federation of Parents and Citizens Associations of NSW (NSW P&C)
- Queensland Association of State School Principals (QASSP)
- Queensland Secondary Principals’ Association (QSPA)
- Queensland Association of Special Education Leaders (QASEL)
- South Australian Secondary Principals’ Association (SASPA)
- South Australian Primary Principals Association (SAPPA)
- South Australian School Parent Communities
- WA Secondary School Executives Association (WASSEA)
- Western Australian Primary Principals' Association (WAPPA)
- Tasmanian Association of State School Organisations (TASSO)
- NT Council of Government School Organisations (NTCOGSO)
- The National Aboriginal and Torres Strait Islander Principals Association
- Parents Victoria
- Save Our Schools
- The Parenthood
- National Union of Students
- ACTU/ Australian Unions
Unions have blasted the $4.6 billion peace deal for Catholic and independent schools announced last month for not being equitable, sector-blind or needs-based.