Lack of proper uniform can make a child feel isolated, it is said.
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Children whose families cannot afford the right school uniform are in danger of being bullied, charities warn.
Too many children are also missing out on school trips because their parents cannot pay for them, they said.
The charities - including Citizens Advice and Child Poverty Action Group - want councils to offer dedicated grants for uniforms.
They say schools should make sure their uniforms are affordable and do not exclude low-income groups.
Citizens Advice chief executive David Harker said: "The costs of taking part in school life can place a great deal of pressure on the budgets of low-income families.
"Not having the money to buy the correct uniform, join in activities outside the classroom, or go on school trips can mark children out as being poor, and lead to them becoming isolated within school.
"It may also leave them unable to fulfil their full learning potential."
Research conducted by the Department for Education and Skills shows the average cost of putting a child through state secondary school is nearly £1,000 a year.
For parents of primary schoolchildren the cost is about £600 a year.
The costs cover uniforms and sports kits, school meals, textbooks and other items.
The director of One Parent Families, Chris Pond, said ministers and local councils should "act now" to make sure children from poorer backgrounds were not excluded.
"It is a bitter irony that a free school system still imposes costs on families," he said.
"Too often single parents are having to say 'no' to a child who wants to join a school excursion or take up extra swimming lessons or a music class."
Government guidance is that it is up to schools and local education authorities to decide whether to have a uniform.
If they do have a uniform, it should suit the pockets of parents in the area.
Many councils do offer help to parents struggling to pay for school uniforms, but campaigners want this to be made a statutory duty.
A report by Citizens Advice in 2001 found that almost one in three local authorities did not offer families help in buying uniforms.