The national weigh-in of primary pupils has begun. In Birmingham, obesity data on children is already being collated.
Every four and 11-year-old will be weighed from next year as part of the Government's drive to curb childhood obesity.
School nurses will weigh reception children, and a mixture of teachers, teaching assistants and nurses will weigh Year 6 pupils.
Health authorities in England are responsible for gathering the data, and in some parts of the country, like Suffolk and Birmingham, weighing has already started. In Birmingham, primary headteachers will be given pie charts showing the proportion of children who are obese.
The move is part of the Government's plans to halt the rise in obesity among children under 11 by 2010.
A Department of Health spokeswoman said the results would give the first detailed national picture of the obesity crisis facing the country.
Ministers have yet to decide whether parents will be told if their child is dangerously fat.
Professor Paul Gately, a UK expert on child obesity, said he believed the move would do nothing to solve the problem of overweight youngsters. He said that the Government was right to act, but that its "piecemeal"
strategies to halt the rise in child obesity by 2010 will fail.
An estimated 2 million children in the UK are now overweight, with a further 700,000 classed as obese and endangering their health.
This is despite government initiatives such as encouraging youngsters to eat more fruit and vegetables and urging schools to provide two hours of PE a week. "Fruit is good for health, but it will never have an impact on weight," said Professor Gately. "Two hours of PE a week is nowhere near enough.
"Improving school lunches will also not make much impact, because most calories are consumed elsewhere. It's a great move to say the Government is going to tackle the problem, but ministers haven't got off their backsides and learned anything about it."
Professor Gately runs the Carnegie weight management programme at Leeds Metropolitan university. Since 1999, its weight loss camps, the only ones in Europe, have helped more than 600 children to lose weight and maintain the loss. He said the camps worked because they are intensive and expensive. "The Government obviously wants to go about it in the cheapest way possible, but it will not get results."
In Birmingham, Year 5 pupils in a third of the city's 275 primary schools will have their weights recorded by the end of this term.
The weigh-ins are part of a range of activities in maths lessons and feature scales with displays which can be read only by teachers, to reduce the risk of children being bullied.
Children are divided into groups and move around the classroom measuring and recording different parts of their bodies, such as the span of their hands.
The project is being led by Deirdre Bryant, a primary teacher turned obesity expert, who said 11-year-olds would be targeted next year in personal social and health education lessons.
* The Local Authority Caterers' Association has warned that new nutritional standards for school meals could push their selling price up by as much as a third, further depressing demand.