Some schools in Wales hoard stockpiles of money in reserve while others are starved for cash, claims a union.
New statistics show £71m went unspent in schools last year while others face a total deficit of £13m, an increase of £3m on the previous year.
Some 28% of secondary schools have recorded a budget deficit, an increase of 7% on 2008-9.
Rex Philips of teaching union NASUWT, said the funding system was not working.
"It is quite outrageous that we face teacher and support staff redundancies while other schools have stockpiles of cash," he said.
The figures show that Monmouthshire faces the most serious challenges, with 28% of schools there recording a deficit.
The council is predicting that 43% of schools will be in the red by the end of this year.
Meanwhile, all schools in Pembrokeshire have a positive balance sheet.
From April next year new regulations will limit the amount schools can hold in reserve.
Secondary schools will be restricted to holding £100,000, while primary schools can keep back £50,000.
However, separate research by BBC Wales shows some schools currently have reserves far in excess of those sums.
It showed, for example one secondary school with reserves of more than £470,000 while another faces a deficit of £491,670.
The NASUWT is calling for changes to the way schools are funded, which it said accounts for wide variations between schools and between different parts of the country.
Local authorities receive money from the assembly government which is passed on to schools using a locally-agreed formula based largely on pupil numbers.
The assembly government will set its budget on 17 November.
First Minister Carwyn Jones has promised to increase education spending by 1% above funds received from the Treasury.