Primary schools are not spoilt for choice when it comes to picking a new head.
There are plenty of deputies in secondary schools to fill headships. But in the primary sector there are barely enough to comply with the old requirement of five applicants on the short-list.
After allowing for posts filled by existing heads, about 1,400 primary schools each year will need to give someone their first taste of power.
There are 13,000 or so primary deputies aged between 35 and 54 who might be expected to fill most of these headships. Assume 20 per cent of them are too new to be looking for promotion and another 20 per cent don't want the job. This would leave between 7,500 and 8,000 deputies, or at best six deputies per headship.
If the 50 to 54 age group is excluded there could be fewer than five deputies chasing each headship. A comparable analysis would show that 45 deputies were chasing each secondary post.
One in three primary teachers over 50 is either a head or a deputy while some primary teachers have become deputies, or even heads, before they are 30.
Only around one in eight secondary teachers is in senior management, and the youngest headteachers are in their early 30s.
The large number of teachers who have left the profession over the past few years means that there were only about 18,000 teachers between 55 and 59 in 1998 - compared with 55,000 aged between 50 and 54.
The relative absence of classroom teachers over 55 goes some way to explain the high percentage of senior staff in this age group. However, government offers to provide funding to allow some heads over 55 to retire early will, no doubt, have the effect of cutting the numbers of heads in this age group.
John Howson is a visiting professor at Oxford Brookes University. E-mail: email@example.com