Headteachers union the NAHT this week said it had uncovered evidence of the "dodgy practice" of schools being "ripped off" in the supply and maintenance of IT equipment.
Speaking ahead of its annual conference in Liverpool, the union has called for a "major inquiry" into the procurement of IT equipment, for fear of schools being treated like an "open chequebook" by companies.
In a report commissioned by the NAHT, schools and teachers have revealed they have been forced into buying equipment they do not need or become tangled in expensive contracts that end up costing the school thousands of pounds more than was needed.
The report quoted forum posts from EduGeek, a website for IT professionals working in schools, which listed a raft of examples of schools being taken for granted by companies. One IT expert said companies exploited teachers' lack of knowledge, charging vastly different sums from one teacher to the next.
The expert said: "I manage a college and several primary schools. The previous IT teacher was charged #163;3,500 (for an empty server cabinet) and #163;145 per network point. I was using the same installer at the same time at my previous college and they charged me #163;600 for the same cabinet and #163;45 per point.
"I also had a local authority-approved electrician try to charge #163;2,800 for five sockets for over-head projectors (the job was completed for #163;345 by another (electrician))."
The news comes just weeks after The TES revealed that a school had spent #163;250,000 on three photocopiers after becoming caught in expensive leasing arrangements.
Richard Spragg, an IT professional who runs a company offering support to primary schools, said he was often staggered by what he saw.
"When I go into schools, there are far too many companies that seem to treat them like an open chequebook," Mr Spragg said.
"Schools bring in IT companies and trust them to do things. But so often, I find that it has not been done properly."
NAHT general secretary Mick Brookes has called for a Which? guide for schools when it comes to IT procurement, claiming that the targeting of schools could become a "major scandal".
Mr Brookes said: "We believe that there should be a major enquiry into the provision of and maintenance of IT equipment."