Low-level disruption in struggling secondaries is undermining teaching standards and diverting school managers from work on raising attainment, according to inspection agency Estyn.
The agency investigated why 26 Welsh schools were lagging behind as part of its low-performing secondary schools programme. It found teachers struggling to cope with low-level disruption from a minority of pupils, which had an adverse effect on class performance.
Their managers were often too busy covering for absent teachers to spend enough time on evaluating the school's work and charting progress. But despite apparently modest GCSE results, many of the schools - serving the toughest and most deprived areas of Wales - were found to be doing well, given pupils' low starting point in literacy and numeracy.
Unions say the report highlights the stresses faced by teachers working in tough schools. Helpline calls from teachers about pupil behaviour were up 45 per cent on last year, according to charity Teacher Support Cymru.
Estyn defines low-performing schools as those "not doing as well as they should" given their circumstances and pupil intake. Most are achieving GCSE results well below the Welsh average, and serving poor urban areas. The inspectors highlight the link between deprivation and results, noting that a fifth or more of pupils are entitled to free school meals in 21 of the 23 secondaries where less than 30 per cent of pupils gain at least five good GCSEs.
Other common features of low-performing schools include high levels of truancy (often condoned by parents); high proportions of pupils with special educational needs and low attainment on entry; high teacher turnover; and sometimes falling pupil numbers.
Inspectors found improving standards in many schools and some examples of outstanding teaching practice.
But they also found evidence of below-par teaching practices, particularly in the marking and grading of work. Some schools were criticised for not spending enough time on basic literacy skills. And others "are only just beginning to get to grips with the full potential of using data to improve pupils' achievement", say inspectors.
The report calls for direct cash aid from the Assembly government to get low-performing schools back on track. It also recommends local education authorities give more advice and help to schools struggling to raise standards. The teachers' union NASUWT Cymru welcomed the "radical" move to directly fund low-performing schools.
Secretary Geraint Davies said increased scrutiny of the schools had led to high sickness levels and low morale among teaching staff, who were clearly under stress.
Dr Heledd Hayes, education officer for the NUT Cymru, said the report was "supportive" of teachers who worked in difficult conditions. She called for smaller classrooms and the universal use of individual learning coaches.
Norah Clarke, director of Teacher Support Cymru, said: "There is undoubtedly a link between pupil disruption and effective learning in the classroom. A zero-tolerance policy is vital to ensure the school environment is safe and does not compromise the well-being and effectiveness of teachers and pupils.
"Last year calls to our support line regarding pupil behaviour increased by 45 per cent. It is encouraging to hear that Estyn has made concrete suggestions on how to tackle this."
Low performing secondary schools 2004-2005, see www.estyn.gov.uk