Arrangements that force parents to buy school uniforms at a premium from exclusive suppliers are facing scrutiny by the competition watchdog.
The Competition and Markets Authority (CMA) has sent an open letter to head teachers, governing boards and suppliers urging them to let parents buy uniforms at the best possible prices.
The CMA said it had received complaints from parents concerned about prices and quality as they bought uniforms for the start of the school year in September.
The letter says some parents in England had been forced to pay up to £10 extra per item of uniform where schools had appointed exclusive suppliers.
The CMA warned that these arrangements "may not be offering parents value for money" and urged suppliers and retailers with such arrangements in place with schools to check they are not in breach of competition law.
It is advising school governing boards to call for a review of uniform arrangements to ensure there is competition between suppliers and retailers and is urging head teachers to listen to parents, heed Department for Education guidance and prioritise value for money when choosing uniform policy.
It has also called on potential suppliers and retailers who are finding it difficult to sell school uniforms because of exclusive supply arrangements already in place to complain to the CMA.
CMA senior director Ann Pope said: "Buying school uniforms can be very expensive and particularly hits low income families and those with a number of children, so it is important parents and carers are able to shop around.
"We urge everyone involved to ensure that they are providing a good service to parents and carers and complying with Department for Education guidance.
"We will continue monitoring the sector and will consider taking enforcement action, if it is necessary."
Sam Royston, director of policy at The Children's Society, said: "School uniform costs can be a millstone around the necks of poorer parents, contributing to a cycle of debt and damaging the opportunities and well-being of lower-income pupils.
"One reason for the high costs are policies that force parents to buy clothing from specialist shops, and prevent them from buying cheaper items from supermarkets.
"We hope the CMA's letter will prompt all schools to take a fresh look at their policies and make sure every parent is given the chance to shop around for the best deal."
A National Governors' Association spokeswoman said: "NGA recognises that school uniform can form a key part of the identity of a school, but governing boards should make every effort to keep uniform costs to a minimum and make sure it is as widely available to purchase as possible."
A PTA spokesman said: "PTA UK supports the CMA campaign and welcomes its focus on the cost of school uniform to help ensure parents get the best value for money possible."
A survey by the Office of Fair Trading in 2012 found 74% of state schools placed restrictions on where uniforms can be bought, leading to parents paying £5 to £10 more for individual items.