Too many children are being let down in the early stages of secondary school, with the transition from primary school often poorly managed, Ofsted has warned.
A report by the regulator found the progress made during Key Stage 3, which normally covers years 7 to 9 when pupils are aged between 11 and 14, is often slow - particularly in English and maths.
Teaching too often fails to build on the gains pupils have made in primary school, it said, with both the leadership and education during this period often a cause for concern.
It suggested that school leaders treat Key Stage 3 as the poor relation of Key Stages 4 and 5, and as a result the deployment of staff and resources is too often skewed towards the upper age ranges.
One in five inspection reports identified Key Stage 3 as an area for improvement.
The report, which used evidence from more than 1,900 inspections, interviews with 100 school leaders and almost 11,000 questionnaire responses from pupils, said teachers are not consistently building on pupils' prior knowledge and skills.
It said the quality of homework in Key Stage 3 is too variable and does not effectively enable pupils to consolidate or extend their learning.
Inspectors also concluded that teaching in modern foreign languages, history and geography at Key Stage 3 often fails to challenge and engage pupils, which impacts on the take-up of these subjects at GCSE.
Head of Ofsted Sir Michael Wilshaw said: "Today's report demonstrates that too many secondary schools do not give provision at Key Stage 3 the priority it deserves.
"Inspectors have found that pupils often leave primary school with good literacy and numeracy skills, confident and eager to learn, but their progress then stalls when they start secondary school.
"The importance of a good start to secondary school education cannot be overstated.
"School leaders need to have a clear understanding of their pupils' achievements in primary school and build on them effectively from the day they start secondary school life."
General secretary of the Association of School and College Leaders Brian Lightman said: "It is disappointing that Ofsted has chosen to use such a negative title for a report which actually contains much evidence of good practice.
"The reality is that schools are working hard on ensuring a good transition from primary to secondary and the education of 11 to 14-year-olds.
"It should also be noted that a vast amount of good practice is not seen by Ofsted because it does not routinely inspect outstanding schools."
Russell Hobby, general secretary of school leaders' union NAHT, said: "We know that this stage is crucial in the development of pupils, not least in ensuring a smooth transition from primary to secondary education.
"School leaders continually work hard to make sure that pupils are supported at the start of secondary education, whilst receiving teaching that stretches, supports and develops.
"The danger is that the high stakes accountability applied to GCSEs and A-levels, along with constant tinkering by government, distracts attention from Key Stage 3.
"It is hard to find the specialist staff to teach this age group, for example, when you are also transforming your GCSE offer and facing a recruitment crisis at the same time. This shortens school leaders' horizons and forces them to focus on the short term."