The Office of Fair Trading has asked almost 30,000 state schools to review their uniform policies to allow parents to shop around for cheaper items.
The OFT's request follows a survey which found that items could be as much as £5 to £10 higher when bought from retailers or suppliers selected by the school or from their own shops.
It said families could save tens of millions of pounds if primary and secondary schools removed restrictions and allowed them to shop around freely.
The study also found that 74% of state schools continue to place restrictions on where parents must buy some uniform items.
In one example, the average price of a secondary school's sweatshirt was £12 under a restricted arrangement compared with an indicative cost of £8 at competing retailers and £5 at a supermarket.
The survey found that schools use a single supplier for a number of reasons, including wanting a consistent, good quality uniform and convenience for parents.
The OFT said schools could still have "smart" school uniform policies without appointing a single supplier.
It suggested that schools could achieve consistency through setting out colour and style requirements in more detail but still allow parents choice about where to buy items.
Susan Oxley, assistant director in the OFT's goods and consumer group, said: "When schools require that uniforms are bought from a preferred supplier or shop it can act as a tax on families, which mostly goes to the chosen retailers.
"However, when families are able to shop around for school uniform items it can drive competition and bring down prices for all. We know schools don't want families to be left out of pocket and we have written to schools across the UK asking them to review their policies and supplier arrangements."