The Office of Fair Trading has launched an inquiry into school uniforms after receiving complaints about high prices and poor quality.
Schools can choose which uniform retailers to use
The OFT has written to 10,000 schools to find out whether their policy allows a choice of where to buy clothes.
The uniforms market is thought to be worth about £450m a year, with most state pupils having to wear them.
The OFT said it would find out whether contracts between schools and firms had an "adverse effect" on consumers.
Governors usually set dress codes, while schools can appoint a manufacturer to produce clothes which are sold directly to parents.
They can also select which outside retailers to use.
The OFT has received complaints of high prices and poor quality at school-nominated outlets and from traders claiming the market is closed to them.
In its letter, it asks whether parents have to purchase uniforms from a designated retailer or the school itself.
The OFT is also assessing whether arrangements have a damaging effect on poorer families.
Chief Executive John Fingleton said : "This study will allow the OFT to see whether exclusive contracts between schools and retailers have an adverse effect on the prices paid by parents, as well as the quality and value of school uniforms."
A spokeswoman for the supermarket Asda said: "We are always getting bad reports from customers who say their school is telling them they can't buy their uniform from us, forcing many parents to pay over the odds for their kids' school clothes."
A Tesco spokesman added: "Parents facing ever-increasing household bills for energy, council taxes and fuel should be able to buy their school uniforms wherever they find the best value."
Liberal Democrat education spokeswoman Sarah Teather said: "Some schools require a certain expensive brand of clothing in order to deter poorer parents from sending their children to the school.
"It's selection by the back door and it's got to be stopped. As schools become trusts and get more independence it's crucial that such tricks are rooted out."