Head teachers representing some 3,000 schools in England have written to their local MPs and ministers calling for a rethink on school finance plans.
They say a new national funding formula, which should give underfunded schools more cash, ignores inflationary cost pressures faced by all schools.
The heads come from 14 local council areas and represent 1.5 million pupils.
The letter comes as the government's consultation period for the new school funding formula closes on Thursday.
The letter has been signed by primary, secondary and special school heads from a number of counties in England including West Sussex, Essex, Cambridgeshire, Norfolk, Suffolk, Devon and Cornwall.