But how valid is it? An investigation of those returning to teaching from a break in service reveals clear evidence that for more than a decade the numbers have been falling.
In 1985-86 the Department for Education and Employment recorded more than 18,300 entrants to full or part-time teaching from "out of service" (not working as a teacher), out of a total of nearly 42,000 entrants in that year. By 1995-96, the latest year that figures are available, returners accounted for just over 11,000 out of the total of 46,000.
Returners account for only 24 per cent of the total in 1995-96 compared with 44 per cent in 1985-86. Part of the reason for this decline in returners may be that these teachers have lost out to new entrants in the competition for jobs. Newly-qualified teacher numbers rose in the same period from around 12, 500 to more than 20,500, as training targets recovered from their historic lows of the late 1980s.
Since the early 1990s was the boom time for recruitment to teacher-training courses, however, such a large number of new entrants is unlikely to be seen again for some time. Changes in the way teachers are recruited mean that many returners may sign up with a supply agency. Such agency staff may not be counted in the figures.
However, the most likely explanation is that returners are just not there in the same numbers as in the past. Former teachers are part of what the DFEE labels the EMPIT (Ex- Maintained now in the Pool of Inactive Teachers). The latest figures showed just 17 per cent of men and 21 per cent of women in the EMPIT were under 40. After 40 the probability of re-entry to teaching from the EMPIT drops sharply to around 10 per cent for men and 20 per cent for women.
Without a pool of inactive teachers the only things to do are hold on to the existing workforce and attract more new entrants.
John Howson is a fellow of Oxford Brookes University and runs an education research company. Email: email@example.com.