The Office for Standards in Education said secondary teachers needed to give parents more detail about children's strengths and weaknesses and say how they measured up against national standards.
Ofsted warned that pupil assessment was good in only a third of secondary schools, and that poor report-writing and a lack of constructive marking were to blame.
David Taylor, Ofsted director of inspection, said: "It's just not good enough to say 'He has had a good term doing satisfactory work.'
"We should be talking about targets, and a sense of the individual things that pupils can work on to really improve."
He is worried reports may become even less personal if staff use computer programs with pre-written phrases.
The Ofsted study showcases 12 schools that assess well. At Dixons city technology college in Bradford pupils check that their annual reports are fair before they take them home and can contest them.
Examples of comments from Dixons' reports include: "She could improve her anticipated grade to A* if she practises sketching graphs from equations" and "He must extend his background knowledge by using secondary sources of information to expand his notes".
The Qualifications and Curriculum Authority has launched an advice page on report-writing on its website, and is inviting teachers to give feedback.
It also urges teachers to write in a clear and straightforward way and to ensure that reports are personalised.
John Bangs, National Union of Teachers' head of education, said: "If reports are not going to be a complete waste of time, then they should be direct and constructive - but teachers must be given the spare time to write them."
"Good assessment in secondary schools" is at www.ofsted.gov.uk publications; "QCA - Report writing" is at www.qca.org.ukcatests 2003sample_reports.asp