Teachers in secondary schools meet them by the score - youngsters who enter their new school with no particular problems or question marks against their names. They were regarded by their primary teachers as average-to-high achievers, and there is no reason to suppose they shouldn't continue to make progress. Yet three years later they are still at level 4.
Somewhere within key stage 3 - apparently in Year 8 - they go off the boil.
Alan Howe, senior director of the key stage 3 national strategy, says: "These children are not low attainers. They're not special needs children. They are potentially high attainers and perfectly capable of doing the work."
So what has happened?
"Unlike lower attainers, whose needs can probably be addressed by a specific programme or single approach, these children's lack of progress appears to have different but interconnected causes," he says.
"Some are truants, some disaffected and mildly disruptive. Some are working hard and dutifully but fall behind without anyone realising it. Some are quiet and timid."
One study found that such pupils often feel lost in the crowd and neglected when they reach secondary school, especially after the more sustained attention they experience during their primary years.
"They don't see the work they are doing as interesting, relevant to their lives, or particularly engaging in terms of the activities they undertake and the teaching styles they encounter," says Mr Howe.
This year, the key stage 3 strategy is focusing particularly on pupils who aren't achieving all they should be - those who enter secondary school at level 3 but have not been identified as having special needs (the low attainers) and those who leave primary with level 4 but are at risk of not reaching level 5 by Year 9 (the non-movers).
Childwall comprehensive in Liverpool is one of 20 secondaries nationally being given the chance to take a flexible approach to the curriculum to help boost the attainment of pupils who arrive with level 3 results. The headteacher, Dewi Phillips, believes his school can devise "an innovative curriculum that meets the preferred learning styles of this group and matches their personal and social needs".
Childwall was chosen because more than a quarter of its pupils enter KS3 with Sats levels of 3. Many come from socially deprived areas and about half are eligible for free school meals. For more than half, reading age is below chronological age.
The school has an energetic management team and enthusiastic staff who have helped Childwall achieve sports college status. They have also established one of the city's biggest sixth-forms in collaboration with two other schools, and raised the numbers going on to university from just one in 1993 to 53 this year.
"What appears to be lacking at key stage 4 and prevents progression," says Dewi Phillips, the headteacher, "are the learning skills and self-confidence to be creative, information-processing skills and the ability to transfer those skills from one subject to another."
The 20 schools in the four-year pilot have been given licence to innovate - particularly in literacy, numeracy, ICT and learning skills. They each receive pound;27,740 a year and will be given five whiteboards, plus support and advice.
The staff at Childwall aim to make learning more child-centred by allowing schemes of work to stem from pupils' own interests. One idea is to teach maths through PE. For example, pupils might time themselves running and make calculations related to their performance. They might also make videos of themselves and work together to find ways to improve.
Another idea is to set up cross-curricular topics. The theme of "spin" could be used to explore dance, the earth's rotation in geography, spin paintings in art and so on. Other ideas include a pond-building project that would incorporate science, humanities, design and technology, maths and English. There are also plans to make kites and investigate steel-making.
Reaching level 5 by the end of KS3 requires a greater degree of analytical, conceptual and interpretative thinking from pupils. Those who struggle feel unprepared for the demands of the KS3 tests - and, because their problems often go unrecognised until it is too late, systematic tracking of their progress is critical so that they can be given extra help.
Planners say there is no evidence that the under-achieving pupils are not bright, nor do they do have special needs. Inspection reports suggest that the quality of teaching is improving - so the finger points at curriculum diet, organisation and timetabling. This is where the pilot schools are hoping to make a difference.
'Tackling Underperformance: a guide for school leaders' comes with a DVD.
HEAD THEM OFF AT THE IMPASSE
* Use practical, visual, oral and kinaesthetic approaches. Hands-on, energetic activities always go down well, and pupils like solving problems that seem relevant to their lives.
* Explain, demonstrate and exemplify new learning. Make the thinking visible
* Support their first attempts at applying a new skill in a new context.
* Provide mentoring wherever possible - mentors can be adults, older pupils or other teachers.
* They may find writing hard, but talking can help them to get their ideas together.
* Set realistic targets - pupils like to feel they are making progress.