The accusation was part of a wide-ranging attack on councils made by David Hart, general secretary, last weekend.
He used his final conference speech to suggest that responsibilities such as admissions and exclusions could be delegated to partnerships of schools and that much more education funding should bypass town halls and go straight to schools.
Mr Hart began the conference in Telford by releasing figures - described as dubious by local government leaders - showing huge rises in the fees that some authorities were charging schools for services such as building maintenance, payroll and personnel.
He quoted the case of an unnamed Midlands primary head, who said his service charges - representing about 10 per cent of his overall budget - had increased from pound;49,700 to pound;60,400, or by 21 per cent in 200506. This was not exceptional, said Mr Hart.
"It is easy to say go to the private sector and they will give you a better deal," he said. "But if you are a small school you haven't got the leverage and they don't want to know. The local authorities are practically in a monopoly position and they are abusing that position."
The NAHT wants such charges to be capped so that they do not exceed schools' minimum per pupil funding guaranteed rise, which this year was 4 per cent.
But Sir Sandy Bruce-Lockhart, chair of the Local Government Association, said that not only could schools buy services from elsewhere, but local authorities were also providing excellent value for money.
Examples from the NAHT included Haringey, north London, which it said had raised property services charges by 146 per cent, governor support by 73 per cent and personnel by 55 per cent.
However, George Meehan, Haringey's executive member for education, said the figures were unrepresentative and in previous years the charges had not covered the true cost of many services.
"Haringey's charges, though not as unrealistically low as in previous years, are still good value for money," he said. "In the case of personnel, for example, charges match those of private- sector providers, some of which are chosen by Haringey schools."