Exclusive suppliers of school uniforms can be more expensive for parents
Schools in England are being told to limit what parents have to pay for things like uniforms and trips.
The amount parents spend on school costs has increased by 4% in five years. Four in 10 say they are finding these difficult to pay.
Such additional costs include uniforms, lunches and out-of-class activities.
Children's Secretary Ed Balls said he was "very concerned" that some schools were not following existing guidance.
Parents spend on average £683 a year on children at primary school and £1,195 at secondary school, the report says.
The Cost of Schooling Report 2007 looks at the cost to parents of uniforms, PE kit, trips, lunch, travel, stationery, extra classes and voluntary contributions.
It found half of all parents felt pressured into contributing towards voluntary school trips.
In addition one in six parents said they had to buy school uniforms from a sole designated supplier or the school itself - a breach of government guidance that such exclusive arrangements disadvantage poorer families.
Mr Balls warned schools that it was "unacceptable and unlawful" to ask for compulsory contributions to school trips which are part of the curriculum.
Uniform contracts with sole suppliers are in breach of government guidelines
Such contracts can be referred to the Office of Fair Trading
Local authorities may offer school clothing grants
Schools cannot oblige parents to contribute to trips which are part of the curriculum or during school hours
Families eligible for free school meals should not pay residential costs of school trips
Source: Department for Children, Schools and Families
The survey found that 10% of primary and 32% of secondary schools admitted they did make compulsory charges for trips which were linked to the national curriculum.
Only 38% of parents knew that they should not pay for trips during school hours, according to the report.
Mr Balls also said he was, "very concerned that some schools risk putting extra pressure on family finances, particularly in the present economic climate, because they are not following the rules on charges".
He wanted to underline schools' legal responsibilities to keep costs down - and said they should do more to publicise financial assistance available.
"The rules on charging are crystal clear and no child should be disadvantaged or penalised because they cannot afford out of class activities.
"It is completely unacceptable for schools to levy compulsory charges for activities, like day trips, which are part of the school curriculum or for families to feel pressured into making voluntary contributions.
Schools 'do well'
The report did say that 83% of parents were happy with the cost of schooling - though this was down from 90% in the last report, in 2003.
However, almost four in ten said they found it quite difficult or very difficult to meet the costs of schooling - up from 27%.
The Association of School and College Leaders said the government should commend schools for keeping costs to parents to a minimum over the last few years.
General secretary, John Dunford, said: "The government knows how difficult it is for schools to put a charging policy into action fairly.
"Ministers want schools to increase the number of out of class visits but they should recognise that having to ask for voluntary contributions and carry out detailed and bureaucratic risk assessments add up to a substantial disincentive.
"Schools do well to run as many trips as they do," he added.
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