The Government's competition watchdog has issued an open letter to all headteachers urging them to recognise that these exclusive deals are "not giving parents value for money".
The CMA found in some cases parents were paying up to £10 extra per item because they were buying from the schools' appointed suppliers.
Anne Pope, the CMA's senior director of enforcement, states in the letter: "Schools should make sure that there is competition for the supply of school uniforms to parents.
"Customers benefit from lower prices, better quality goods and services, new and innovative products and greater choice."
The CMA has also encouraged other suppliers and retailers who are struggling to sell uniforms to get in touch.
These retailers, including supermarkets such as ASDA, Tesco and Aldi, offer similar clothing to suppliers appointed by schools, but for a much lower price.
A CMA spokesman told The Guardian: "This letter is primarily addressed to state schools, including academies and free schools. But we do also encourage fee-paying schools to review their arrangements."
Pope, added: "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."
The CMA also issued a call to action for headteachers asking them to ensure they take on board parents' and carers' views on the prices of school uniform, and "prioritise providing value for money" when selecting school uniform suppliers and retailers.
David Burgess, chair of The Schoolwear Association said while price is important, the first priority is for children to have good-quality uniform as they wear it for up to ten hours a day.
He said: "We are anxious that the focus on school uniform should not be solely on price at the expense of poorer quality.
"Well-made, long-lasting uniform is not only better value in the long term but also the most environmentally sustainable option.
"There are some benefits to sole supplier arrangements, particularly because a single supplier is dedicated to the school and required to stock uniform all year round in all sizes.
"When there are two or more suppliers, that sense of duty diminishes, and the school has less power to demand best value and service levels."
Burgess said if a school decides on a sole supplier arrangement, they should put contracts out to tender where quotations can be compared on quality, service levels, year-round stocking, ethical sourcing and price.
He also said The Schoolwear Association has lobbied the government to remove VAT on school uniform and to enable a voucher scheme, similar to childcare vouchers.