Few school reports explain how pupils can improve their work, the Office for Standards in Education said this week.
Reports are generally useful and many are of good quality, but there are "unacceptably wide variations within schools," said OFSTED in a 35-page study that has divided teachers' unions and education pressure groups.
According to OFSTED, most reports to parents gave a clear and often detailed analysis of achievements, especially in English and mathematics in primary years and in English in secondary years.
But it added: "However, more than half of the reports to parents in all years fail to diagnose weaknesses in the pupils' understanding and skills in curriculum subjects, and thus fail to make it clear what the pupil has to do to improve.
"Pupils are not well supported in developing skills of self-evaluation where teachers fail to report frank judgments about the quality of work."
The Campaign for State Education said the criticism of unacceptably wide variations reinforced its own concern which it had repeatedly expressed to politicians.
Last year the group called for parents to receive two reports annually, one of them a brief update. This is common in Scotland where diagnostic testing is done throughout the year.
But Arthur De Caux, senior assistant secretary of the National Association of Head Teachers, said: "I think teachers will be very, very angry at this report."
Staff had come a long way from the sparse reports of 20 years ago which were often parodied as "satisfactory but could do better", he said.
Workload and time had to be considered in the search for a perfect system, he said. Too much work and "it will just sink under its own weight".
"I would favour reporting much earlier in the year when there is still plenty of time to build on the reports and have an update towards the end of the year."
Mr De Caux added that the national curriculum levels were supposed to show parents how well their child was doing and what they should be aiming for next.
OFSTED said most schools miss the chance to use reports to raise standards. Only a minority make it clear what a pupil has to do in order to improve.
"Though most teachers are prepared to link achievement with efforts and attitude, few provide clear information relating to pupils' attainment to norms for the age group, so parents are often given the false impression that standards are better than they are."
OFSTED also said that over-detailed reporting of the national curriculum often "obscures rather than illuminates"; that many reports are "unduly positive and fail to make constructive criticism"; and that more "clarity and rigour" is needed for setting targets for pupils' work in Records of Achievement.
The National Union of Teachers said OFSTED's study "lacked balance". It said it was wrong to criticise teachers for failing to compare their pupils' performance against national norms when the tests which were to produce those norms did not take place during the period of OFSTED's report.
The Liberal Democrats blamed lenient reports on the Government's market-based education system which was forcing schools to "produce a certain standard" to keep parents happy.
But the Labour party said: "We must learn from this OFSTED study to ensure that the partnership with parents is not one of empty praise but of meaningful dialogue to raise standards in every school in the land."
About half of the 222 schools visited by HM inspectors last year encouraged pupil attendance at parents' evenings.
OFSTED said the active involvement of pupils often leads to realistic action plans, but adds: "In some interviews, however, embarrassed pupils were ignored. There was no evidence of teachers or parents being inhibited in their discussions by the presence of pupils. On the contrary, some difficult issues, such as shyness, were discussed openly."