But since one in three teachers in Germany is over 50 and more than half will reach retirement by 2010, the trend is causing headaches for the 16 German states, which pay teachers’ pensions.

Most teachers are employed as civil servants, and can retire early with full pension rights if certified unfit for work by an official doctor.

A survey in industrial North Rhine Westphalia found that many people are using this right: only 3 per cent of teachers serve until 65 - the average retirement age is 58.

This pattern is similar in other states, especially in western Germany. “The average age of retirement is rapidly steering towards 50,” the prime minister of Schleswig Holstein said recently.

The federal interior ministry has calculated that the cost of providing pensions for civil servants, including teachers, will double by 2008. By 2022 spending on pensions will have become intolerable, it says.

The ministry wants to lower the rates of pay rises for civil servants after 2000 and channel the extra money towards pensions. Some politicians also favour cutting pension payments, which are 75 per cent of a person’s final salary.

A recent report by the Conference of Education Ministers (KMK) investigated strategies for preventing early retirement. The most effective way would be to tackle the root cause of early retirement - “teacher burn-out” - and rehabilitate teachers, it said.

The KMK suggests ageing teachers might also be encouraged to stay if they were allowed to work part-time and receive part-salary and part-pension payments.

Teachers who want out of the classroom could also be transferred to other, lower-grade, jobs within the profession, it said, with the states reimbursing the salary difference so that employees did not lose pension entitlements.

But some individual states are now taking a harder line: they are beginning to cut civil-servant status for teachers, as a cheaper, more flexible, employment option.