Struggling probationers are having a "horrific" effect on entire schools that can leave headteachers "devastated", according to startling new research.
It identifies a small minority of teachers in their induction year whose negative impact can ripple right through a school and beyond, but also finds that even the best probationers pull senior staff away from other duties.
The distress caused by poor probationers prompts calls from teachers for universities to ensure their worst students are discouraged from entering the profession, although there is also evidence that schools do not provide the right support during induction years.
The findings by Ian Matheson, educational planning and research officer at the General Teaching Council for Scotland, emerge in a draft paper examining the support provided to headteachers and mentors (officially known as supporters) after probationers arrive at a school. They are based on focus groups in seven authorities and form the second phase of a long- running research project, which began in 2005.
The study acknowledges that most probationers complete their induction year with "relatively few traumas", and can bring "zest" and "dynamic" approaches, as well as a command of formative assessment and new technology.
Some headteachers and mentors say mentoring is also a good opportunity for teachers' own continuing professional development.
Dr Matheson told The TESS that at least 97.5 per cent of probationers who reached the end of the year would achieve full registration, although no figures were available for the number dropping out.
But his interpretation was sharply criticised by Jim Conroy, dean of education at Glasgow University. He stressed that the minimum 97.5 per cent success rate of those completing the induction year was "extraordinarily high".
Professor Conroy added that postgraduates spent half their time in schools. At Glasgow University, schools had an equal say in whether students were up to scratch. He also bemoaned a tendency, which he said had emerged in all areas of education in the last 25 years, to concentrate on discrete performance indicators rather than performance "in the round".
But in the focus groups, which took place in 2008 and involved about 100 heads and mentors, there were 15 references to the negative - even "horrific" - impact of supporting a struggling probationer.
One said: "That girl was damaging my children every time she walked through the door and that's why, after five weeks, she was never alone with the class. It was nothing short of horrendous and, professionally, it devastated me."
Each week required parental meetings to deal with complaints, and staff were still trying to "repair the damage" one year after the probationer had gone.
One head said things got to "crisis point" when a probationer's needs meant sacrificing time with the pupils.
The ramifications of a failing probationer were far-reaching, said another: "It may take the whole school, indeed the whole authority, to do anything meaningful."
In five of the seven focus groups, questions were raised about universities' assessment of students' competence. "Somewhere along the line, someone is not putting their head above the parapet and saying, `This isn't for you and you really need to think seriously about it,'" a headteacher said.
The report states that, even where a probationer is doing well, "the demands on supporters are often considerable, adding to their workload, often to their working hours, and sometimes making it difficult to give sufficient attention to other responsibilities".
Mentors, in particular, were inclined to say that the half-day a week allotted for supporting probationers was not enough.
The report also finds that "expedience" rather than the most appropriate qualifications often decides who fills the role, especially in secondary schools. Depute heads and principal teachers were usually chosen, partly because headteachers wanted to allocate the half-day to someone who did not have classes.
Only six mentors in four authorities reported having volunteered for the role. One said "there was no consultation at all", another that "you're not in a position to choose".
Ken Cunningham, general secretary of School Leaders Scotland, took a cool view of the findings, saying more information was needed before coming to a judgment. He added: "Whatever the flaws - and it's certainly important to address them - the present system is better than what went before."
He suggested some of the difficulties could be a reflection of the current climate where staffing levels were coming under severe pressure.