Public health experts are calling for "fat letters" – in which teachers write home to parents to tell them their child is overweight – to be scrapped or reformed.
The Royal Society for Public Health said parents did not find the information useful and only half knew why their children were being weighed.
It said that if such notes were judged essential, parents of obese children should be "contacted by telephone prior to receipt of the letter".
And it called for other forms of support, such as healthy food vouchers or better access to after-school activity clubs.
The National Child Measurement Programme (NCMP) measures the height and weight of children in reception class and Year 6 to assess overweight and obesity levels in English primary schools.
More than 1.1 million children were measured in 2013/14 and had their body mass calculated.
Over a fifth (23 per cent) of children in reception were either overweight or obese, while 10 per cent were obese.
In Year 6, 34 per cent of children were either overweight or obese, while 19 per cent were obese.
The Royal Society for Public Health polled 678 parents of children aged 18 or under and found only 49 per cent were aware of the NCMP.
Half (51 per cent) understood its purpose, while 20 per cent had received information as a result of the programme that had been useful in helping their child lose weight.
The society said that the programme needs better integration with other initiatives such as Change4Life and, if the letter is kept, for guides on diet and exercise to be included with the information sent to parents.
Shirley Cramer, chief executive of the Royal Society for Public Health, said: "Parents also need to be provided with support, and our calls to reform the 'fat letter' are intended to make better use of this.
"Our research finds that only one-fifth of parents find the 'fat letter' useful and we believe that the letter should be seen as the beginning of a dialogue with parents, not simply flagging whether their child is obese."