The schools funding gap between England and Wales has widened further this year, new official figures show.
Even more of the money intended for Welsh schools will stay with councils next year, according to Welsh Assembly statistics.
Denying teachers the cash will have "huge" implications for the country's primaries and secondaries, experts and unions have warned.
In 201011, just 74.6 per cent of the money given to local authorities to spend on education will make its way to heads, driving up the difference in per-pupil funding between the two countries.
TES Cymru has also learnt that education funding is being spent on areas such as culture rather than in the classroom.
From September, Welsh councils plan to give heads pound;4,054 per pupil, pound;344 less than their English counterparts. This gap has grown from pound;268 this academic year.
In 200910, 75.7 per cent of the money local authorities received to spend on education was given to headteachers. In the next academic year, this will fall by 1 per cent.
"We have this double whammy of education funding being spent on other things, and then councils keeping too much of it centrally; the effects of this in the next academic year will be huge," said David Reynolds, professor of education at Plymouth University.
Figures show that council officials on Anglesey keep the largest proportion of funding back from schools, passing on just 67 per cent of education funding. Schools in Merthyr Tydfil do best, with 81 per cent of funding handed on to them.
Before devolution, Welsh schools received around 84 per cent of education funding.
First minister Carwen Jones has pledged to increase school spending by at least 1 per cent above the block grant Wales receives from Westminster, warning ministers that they must "claw back" cash from other government departments.
Last year, schools in Wales ended up pound;500 worse off per child than their English counterparts. Professor Reynolds predicts that this funding gap will widen.
Teacher and headteacher unions in Wales have pledged to hold Mr Jones to his pledge.
"We have marked his card and will be looking for changes to have happened by next year," said Phillip Dixon from the Association of Teachers and Lecturers.
"The worry is that the Assembly have also been warned by the Westminster Government that cuts are on the way. We will hear in November if that will affect frontline services."
David Griffiths, head of Peterston Super Ely primary in the Vale of Glamorgan, said he cannot afford to employ all the support staff he needs. The school building also needs "drastic" improvements and children are short of space.
"It's so frustrating that school funding is watered down at every stage before it gets to us," said Mr Griffiths, Wales president of the National Association of Headteachers.
"All we want is for every child in Wales to have the same opportunities as those in England. We will be holding Carwen Jones to his pledge; this situation is of grave concern to us."
A Welsh Assembly Government spokesman said: "We accept that in general terms there is a (per-pupil) gap, but spending in Wales has held up well compared to many of our international competitors.
"We have taken decisions on the basis of what is right for Wales, not by reference to developments in England."