Pupils often failed in early secondary school
Too many pupils in the early years of secondary school are being failed, as resources are skewed towards the upper age groups, according to Ofsted.
Pupils who leave primary school keen and confident to learn all too often stall when they start secondary, England's schools inspectorate says.
It urges head teachers to make this phase of schooling, known as Key Stage 3, more of a priority.
Schools focus on GCSE and A-level years because of their importance, heads say.
The Association of School and College Leaders general secretary Brian Lightman said: "Schools have no choice but to focus more of their resources in this direction, while balancing this demand against the need to give 11- to 14-year-olds a good foundation at secondary level."
The new Ofsted report - Key Stage 3: The wasted years? - explores whether students are given enough support to make a good start at secondary school.
Based on evidence from 1,900 inspections, interviews with 100 school leaders, 14 school visits, and 11,000 pupil questionnaires, the report says too many schools treat the first three years of secondary school as the poor relation of KS4 (GCSEs) and KS5 (A-levels).
KS3 is identified as an area for improvement in a fifth of inspection reports.
Inspectors also found that during KS3:
•pupils' progress is often slow, particularly in English and mathematics
•teaching of modern foreign languages (MFL), history and geography often fails to challenge and engage pupils
•pupils are distracted from learning by low-level disruption, particularly in MFL lessons
•teachers do not consistently build on pupils' prior knowledge and skills
•some schools are not using pupil premium funding effectively
•the quality of homework is too variable.
Chief inspector of schools Sir Michael Wilshaw said the report gave cause for "serious concern" as too many pupils arrived in secondary school keen to learn but their progress then "stalled".
Sir Michael singled out lessons in modern foreign languages, history and geography as often failing to ensure pupils had the confidence and enthusiasm to progress.
"It is, therefore, no surprise that there is a low take-up of these subjects at GCSE.
"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.
"Key Stage 4 results will not improve until Key Stage 3 is given a greater priority by school leaders."
The Department for Education said it was vital all pupils received an excellent standard of education at every stage in their school career.
"We will not tolerate a single day wasted," said a spokesman.
"That is why our new Progress 8 measure will change the way that we hold secondary schools to account, by measuring the progress that all pupils, including the most able and those who have struggled, make throughout Key Stage 3 and 4."
Russell Hobby, National Association of Head Teachers general secretary, welcomed the report's focus on long-term planning in schools.
"Too often in the past, Ofsted has focused on the current year's exam results rather than investments in areas like Key Stage 3 which would only show up in years to come," said Mr Hobby.