A high turnover of teachers can have the same negative impact on a student's attainment at school as an impoverished background, US researchers have found.
The damage to children's test results happens when 40 per cent of teachers in a year group leave within 12 months of each other, according to a large-scale analysis.
And the impact is felt by all children, not just the ones whose own teachers have left the school, academics from Stanford, Michigan and Virginia universities discovered.
The researchers looked at the results of 850,000 children between the ages of 8 and 10 in New York elementary schools, over an eight-year period. The resulting report was highlighted this month by the American Educational Research Association as being one of its most-read studies of the year.
Students in schools where all teachers left during the same academic year suffered the most, but a significant impact was also felt by children at schools with lower turnover rates. "Teacher turnover harms student achievement," the academics said. "More specifically.students in grade levels that experience 100 per cent turnover have lower test scores.as compared to grade levels with no turnover at all."
The impact on students in schools with 40 per cent teacher turnover, compared with no turnover at all, was equivalent to the difference between the expected achievement for a child who was eligible for free school meals and one who was not, researchers said.
In schools with high levels of teacher turnover, even those students whose own teachers did not leave performed significantly worse than students in schools with high teacher-retention rates, according to the research.
The academics said: "One possibility is that turnover negatively affects collegiality or.trust among faculty. Or perhaps turnover results in loss of institutional knowledge. that is critical for supporting student learning."
Russell Hobby, general secretary of the NAHT headteachers' union in the UK, said that moving between jobs was a normal part of a teacher's career progression. "It's accepted practice to give quite a large degree of notice compared to other sectors. That helps to plan ahead," he said.
"But things happen in people's lives that they can't always predict. Their partner may have to move around the country to find work. They may need to look after a parent. And there's maternity leave."
Nonetheless, high turnover rates needed to be carefully managed, he added. "The effect on other teachers, particularly if someone leaves without much notice, is that they have to fill in for other people," he said. "The supply cover might not be enough. And it can have an impact on team planning as well.
"But the closer the school works as a team, the more that teachers can support a new teacher or a supply teacher and maintain continuity."
Supportive school leaders, he added, discussed career development with staff members, helping them to plan ahead. "Also, make sure your school is a good place to stay," he said. "If you treat people properly, they're more likely to stick around."
Just last week, England's chief inspector of schools Sir Michael Wilshaw described the dropout rate of new recruits to teaching in England as a "national scandal". About 40 per cent of teachers left the profession within five years, often because they were unable to cope with poor student behaviour and received insufficient training, he said.
Sir Mike Griffiths, former president of the Association of School and College Leaders and headmaster of the high-performing Northampton School for Boys, said that very high turnover could make it hard to "protect the ethos of the school", but that having little change brought its own problems.
"Static staffing is not great, either. You always want a healthy turnover of staff. You do need new blood, new ideas. Just fresh faces," he said. "People have always moved on, because of their careers. But, as with most things in life, it's about getting an appropriate balance between stability and the new."