They issued new guidelines, warning schools not to let homework interfere with after-school sport, music and clubs.
David Hart, general secretary of the National Association of Head Teachers, said there was a danger that schools would be pressurised into setting too much homework, after recent controversy on its value. He has written to all his members about government guidelines that set out how much homework should be given from the age of five to eleven.
Since ministers published their advice, research has suggested that homework for the youngest pupils does not improve performance and may be harmful. Academics from Durham University found that pupils who did homework only once a month scored better in national tests than those who did it more often.
David Blunkett, the Secretary of State for Education and Employment, retorted that critics of his homework policy were revealing "blatant elitism dressed up as well-intentioned liberalism".
Heads say schools need to find a sensible balance. Mr Hart said: "The real danger is that the Government's message is being distorted and its understandable reaction to the Durham report may imply that homework is the be-all and end-all.
"After-school homework clubs to support pupils from deprived backgrounds should not prevent these pupils from taking part in other after-school activities which are just as important."
Government guidelines say that five-year-olds should spend an hour a week on homework and eleven-year-olds half an hour a day. The oldest secondary pupils should do up to two and a half hours a day.
Heads say that quality is more important than the precise amount of time spent on homework. They emphasise that schools are not obliged to keep to the Government's times and suggest they should be flexible.
A good homework policy, says the new code, should explain to parents the purpose of tasks and what will happen to the work. For example, research may not be marked but contribute to information collected by the class. Homework should not generate "vast amounts of marking".
Pupils should not spend too much time at home finishing off work they started in class. As they become older, the main purpose of homework should be to encourage independent learning.
Mr Hart said: "Homework has a key role to play in raising standards. The precise amount of time spent on homework is much less important than the quality of the tasks set and the way they are planned to support learning.
"The Department for Education's suggested guidelines are a useful guide for schools to follow. The really crucial issue is the need for homework policies to set out clearly the purpose of homework and the expectations placed on pupils and their parents in order to make homework as effective as possible."
Schools' homework policies will be set out in home-school agreements due to be sent to all parents nest month. Parents should:
n provide a peaceful place for homework;
n support teachers in emphasising its value;
n praise pupils when they have done it;
n check deadlines are met.