New government guidance was issued earlier this month to ensure that 80 per cent of these age groups are weighed this school year.
But in its 2004 report on obesity, the parliamentary Health Select Committee recommended that all schoolchildren effectively be screened.
They should be routinely weighed, the results fed back to parents, and overweight and obese children offered specialist treatment, it said.
But there is little evidence to show that preventive approaches or current treatments actually drive down children's obesity in the long term, say the authors.
When they systematically assessed the published and unpublished evidence on the effectiveness of either weight monitoring or screening for picking up and treating obesity, they found none.
Instead, the research focused on the accuracy of weight monitoring for diagnosis. And few studies had been done on the attitudes of parents, children, or healthcare professionals to it.
Weight monitoring is useful for gathering information on obesity trends and informing how money should be spent, say the authors. And it could also be useful for assessing the impact of school initiatives to improve children's diets and lifestyles.
'However, the value of moving from population (weight) monitoring to screening to identify and treat individual children remains at best questionable,' they say.
'The effectiveness of treatment is currently doubtful and the potential harms of either monitoring or screening are poorly researched,' they conclude.