A state school in Waterlooville, Hampshire, has been accused of potentially creating a "back-door selection" system by introducing a compulsory 'eco-friendly' uniform costing about £100.
Oaklands Roman Catholic School in Waterlooville has introduced the uniform made from recycled bottles which can only be bought from the school or from the Schoolwear Shop in nearby Havant.
Other schools also have some degree of exclusivity, where logoed polo shirts or jumpers can only be bought from the school or one shop.
MPs have raised concerns that such expensive uniforms could deter poorer families from sending children to their chosen school.
Government guidance says parents should be able to buy uniforms from several places, not just from the school or a single supplier.
Angela Ferguson, from Parentline Plus, said: "Some schools allow uniforms to be bought from many different suppliers so parents can really shop around for prices.
"But that's not the same everywhere. School uniforms do add up, especially if you have more than one child.
"Parents find it difficult to find the cash, especially in a recession. It is a lot of money."
A former schools minister, Sarah McCarthy-Fry, the Labour MP for Portsmouth North, said: "There is no reason why schools can't allow parents to buy blazers for supermarkets and sow on the school badge. And they should be allowed to buy shirts and trousers from wherever they like."
Parents have pointed out that supermarkets like Tesco can supply entire uniforms for only £3.50.
The new uniform at Oaklands is compulsory for Year 7 pupils this year, but by September 2010 all pupils from Years 7 to 10 will have to wear it.
Jane Hopkins, whose daughter Lily, 12, attends the school, said: "Like me, most parents are annoyed about it. It almost looks like a private school uniform and doesn't look as comfortable."
Another parent, Carole Harrison, who has two children Jaimez, 14, and Natazjah, 13, said: "We're a low income family but we're not on benefits so we can't claim any help. I've been putting some money in the kitty so we can afford the uniforms. They do look smarter but I'm not happy about the extra expense."
Matthew Quinn, the school's head teacher, acknowledged the uniforms were "quite pricey".
He said: "If parents are short of money Hampshire County Council has a uniform grant and the governors have set aside some hardship money for parents who find it difficult.
"We felt the uniform we had was dated and the stronger colours would better reflect the image of the school. There was significant consultation with parents, students and staff.
"We made a decision and that's the decision we decided to make.
"Boys can buy their white shirts and trousers from other sources and we feel we're within the spirit of the guidance."
The government guidance on school uniforms is non-statutory, and parents can only complain to a school's governing body.