Jim Knight, the schools minister, wrote to all headteachers this week clarifying that they did not need to seek parental permission for next year's weigh-ins, though families will still be allowed to opt out if they wish.
Last year many schools chose to obtain parental consent, despite not being obliged to do so. As a result large numbers of pupils were passed over, say nurses.
Other changes set out in the guidance include more scope for teachers and support staff to help with weighing and measuring, and a greater onus on schools to support the scheme. Sharon White, a professional officer at the Public Health Nurses Association, said: "Co-operation was a major prohibitive factor last year. We did not have good access to young people or proper areas to work in."
Mr Knight has told heads: "You will be asked to provide class lists, identify a suitable place in the school I and facilitate escorting children to and from the measurement location."
Bruni de la Motte, national officer for education staff at the public services trade union Unison, said teaching assistants would be prepared to help with weigh-ins so long as they were supervised.
The Government aims to weigh and measure 80 per cent of children in reception and Year 6.
Parents of large pupils are being unfairly penalised with an extra tax bill of pound;35 million a year, according to the Schoolwear Association. Some 40 per cent of school uniform sales now attract VAT because trousers for waists bigger than 28 inches and skirts for waists of more than 26 inches are classed as "adult sizes".
The association is campaigning for all badged uniform items to be exempt from tax.