Nearly a third of education authorities provide no help with school uniform costs for families on low incomes, says the Citizens Advice Bureau study.
It cites the case of a boy, living with his mother on income support, who was threatened with exclusion because he was wearing the wrong-coloured trousers.
In the past 10 years, discretionary funds have declined and, in most cases, is less than half the pound;105 cost of the cheapest outfit.
John Dunford, general secretay of the Secondary Heads Association, said schools would only exclude if a pupil's lack of uniform was a blatant disregard for school discipline.
The charity has called for a legal duty on local authorities to provide grants, with the money earmarked in the education standard spending assessment.
It is estimated that parents spend pound;1 billion a year on school dress. A TES check on uniforms found prices could vary greatly. The kit for Grey coat Hospital girls school, Westminster, supplied by Sloane Square's Peter Jones, costs a minimum of pound;222.85.