Schools are paying up to 500% more for equipment because of regulations, principals told the committee.
Head teachers gave numerous examples of goods which were more expensive to purchase through the central procurement process than elsewhere.
The Education Authority runs centralised procurement services for all controlled and maintained schools, covering equipment and contracted services such as maintenance.
One Belfast headteacher told MPs how lighting equipment was 490% more expensive when sourced through a procurement list.
Jo McColgan, principal of Ashfield Boys High School, said: "I needed to purchase lighting equipment for an A-level class. To go through the procurement list, it was £320.
"I was able to get the exact same piece of equipment with extras, through Amazon, for £54 but I was not allowed to buy it. I had to go through the list.
"To give you another simple example, I needed a new washing machine for hospitality and home economics. To go through the procurement list it was £349. I was able to buy a better machine from a local well-known high street supplier at £190.
"Those two pieces alone would have had a saving of £425, but I am not allowed to do that.
"It gets worse. Ink cartridges for computers and printers I was able to get for £6,000 less than procurement."
The cost of maintenance services was similarly high, MPs were told.
Another principal said the cost of fixing a window is £30 for a local contractor, but £200 through the EA contractor.
In its recent report on the Financial Health of Schools, the Northern Ireland Audit Office reported similar concerns.
One school said it was able to carpet a classroom from parent/teacher funds for considerably less than the price paid for other classrooms via central procurement.
Another school said it had arranged its own cleaning contract and halved the price of the EA contract, saving around £30,000.
Not all schools shared the same view, however.
MPs spoke to trade unions who suggested that decentralising procurement responsibilities would substantially increase schools' workloads.
Witnesses acknowledged that there were some areas of procurement where bulk purchasing or standardisation of a product or service were beneficial. This included C2K, the IT network for Northern Ireland's schools.