The White Paper on public health, published on Tuesday, said schools will be expected to educate children about healthy living, provide free fruit and healthier meals an`d encourage pupils to take part in sport both within and outside school hours.
The paper also sets out plans to develop more specialist sports academies and strengthen protection of school playing fields.
Schools' efforts to educate pupils about healthy eating and improve meals will be monitored by the Office for Standards in Education.
Children will be encouraged to cycle to school. At home they will be protected from junk food advertising before the 9pm watershed by voluntary curbs monitored by regulator Ofcom.
This week's announcement comes amid increasing pressure for the Government to act over children's health.
Obesity rates among six to 15-year-olds trebled from 5 per cent in 1990 to 16 per cent in 2001, prompting experts to warn that this generation could be the first to die before its parents do.
Earlier this year the TES Get Active campaign was joined by more than 700 schools and was supported by national figures from the Duke of Edinburgh to TV presenter Gaby Roslin.
A National Union of Teachers spokeswoman said: "We were hoping for a total ban on advertising of junk food aimed at children, while the Government will only consider this after 2007."
She said that schools had worked hard to encourage better school meals and called for more resources to run extra sports activities. The Government has already announced a pound;1 billion for school sport.
Dave Prentis, general secretary of the public-services union Unison, welcomed the "common sense route" to better health.
However, he said: "There is no point in banning TV advertising for junk food aimed at children, when that is what they put on their plates every lunchtime in school."
The White Paper also includes measures to improve the sexual health of teenagers and plans to ban smoking in the workplace by 2008.
The Government estimates that one in 10 sexually active young women may be infected with chlamydia, a sexually-transmitted bacterial infection that can cause infertility in women.
The move to outlaw smoking in most enclosed public areas such as staffrooms, and offices and factories is supported by the NUT. There will be wide consultation on the Bill, but reports suggest even smoking rooms could be under threat.