Heads trying to balance the books should act as if there will be no public spending cuts, their union has advised.
Members of the NAHT have been urged to plan to keep the same number of teachers and services, or else risk "scoring an own goal" ahead of cuts widely predicted after the 201011 financial year.
The new Government has already started to make savings, with public services - including education - facing cutbacks. Last week, it was announced that Becta, the education ICT quango, and the curriculum development agency the Qualifications and Curriculum Development Agency were both for the chop.
But despite these pressures, new figures released last week show schools employed a record number of people this year, despite the ongoing economic difficulties. The biggest increases were in teaching assistants and administrative staff.
NAHT bosses believe cuts in education could be worse if heads start to identify savings themselves rather than wait for politicians to wield the axe.
"Our advice to them is to plan as if there are going to be no cuts," said general secretary Mick Brookes. "Heads know how much they have to spend in the 201011 academic year, and if they don't use it all and put the money in reserves, they will score an own goal."
Workforce statistics released by the Department for Education show the number of teaching assistants has risen by 9,000 in the past year to 190,400 and the number of administrative staff has increased by about 1,000 to 72,300.
The number of secondary teachers has fallen by some 3,000 to 209,400, reflecting the fall in the number of pupils in that age group. Primary teacher numbers have risen by 2,500 to 201,000 because of the growth in the birth rate earlier this decade.
Both Mr Brookes and John Dunford, general secretary of the Association of School and College Leaders, have defended the growing numbers of non-teaching staff now working in schools at a time of financial austerity.
"The inclusion agenda has meant an increase in children who need support, so if the Government targets teaching assistants as an area in which savings can be made, they will need to look at their own policies as well," Mr Brookes said. "You can't take this help away from children and expect them to thrive, and it will also have a knock-on effect for other pupils - for example, more disruption in the classroom."
Mr Dunford said heads were employing extra staff to take non-teaching tasks away from teachers.
"They are helping them use their time to focus on teaching and learning," he said. The ASCL leader predicts the number of secondary teachers will fall even further when heads have to struggle with smaller budgets next year.
The changing demography in schools has already begun to affect teacher training. The Training and Development Agency for Schools (TDA) had to increase the number of primary places because of the rise in the number of pupils. But secondary places have been cut.
Patrick White, a sociology lecturer at Leicester University, said the TDA would need to rethink selection procedures for training courses if the population boom continues.
"For shortage subjects, it's going to have to be less rigid," he said. "If they need more teachers in the future, they are also going to have to increase the number of training places."