The second is to be able to show that you have built up positive experience, not just in your own subject area but across the school as a whole. The purpose of leadership is to make things happen, and one of the key skills you need is the ability to define objectives and progress in a wider context. It doesn't matter what you have done outside your subject area - it could be behaviour management, curriculum review, school-industry links, the timetable or whatever - as long as you can show that it has helped you to identify a means of school improvement and to work with others towards achieving it.
The third precondition - and you are half-way there if you can tick the other two - is to be or have been a deputy head. Outside the independent sector, experience as a deputy is an almost universal prerequisite for headship. It has to be the right experience, though. If you are lucky or shrewd, you will work with a head who sees deputy headship as a training ground for promotion and who delegates real trust across a wide spectrum of responsibilities.
Don't allow yourself to be tagged "curriculum deputy" or (worse) "pastoral deputy". Those two roles should go hand-in-hand. Individually, they will side-track you away from the school's core function.
Above all, don't get bogged down with tasks. Many deputies work appallingly long hours. You have to remember that your job is to make the school work more smoothly and more effectively. That doesn't mean doing everything yourself. Delegation - the sharing of tasks, responsibilities and kudos - is as much a part of your role as it is of your head's. It needs practice and training.
Once you are a deputy, it is time to take that training seriously. The new National Professional Qualification for Headteachers (NPQH) will be piloted regionally from January 1997 and should be available nationally by next September.
It will offer a two-track provision: an evaluation of a potential head's professional strengths and needs at an NPQH assessment centre, and then a training programme co-ordinated by an accredited regional NPQH training and development centre, focused largely on school-based activities and assessed by written assignment, an individual portfolio and interview. Depending on prior experience and skill, the qualification should take between one and three years.
Details of funding and selection are not yet clear, but there is little doubt that the NPQH will be the model for all new headship training. Some people see a danger here of a sort of identikit approach. The great advantage, however, is that there will be a common agenda for all training, in the form of the proposed National Standards for Headship. If you are serious about becoming a head, you will need to match your current experience and know-how against these standards (you can get a draft copy from your local education authority or from the Teacher Training Agency) and begin now discussing with your head how best to fill the gaps.
One word of warning. Taken cold, this list (which includes key areas for development and assessment, essential skills and abilities, and required knowledge and understanding) makes pretty forbidding reading. You need to remember that nobody gets top score on all the items. Besides, you have already mastered many of them. The remainder are part of your entitlement to professional development inside your school and your commitment to professional study outside it. Your LEA advisory service, local higher education institutions, the Open University and your professional association will all advise you.
Not everyone who gets an NPQH will go on to headship and, to begin with, not every new head will need one. The probability is, however, that those who do have one will be at an advantage.
Meanwhile, it helps to be mobile. In England and Wales together, about 300 scondary headships are filled each year. A significant number of them - not always the most difficult - are readvertised, and governors report that the field of firm applicants is small and shrinking. Sadly, not all deputies think that the game is worth the candle. For those who do, however, headship now is certainly more attainable.
One thing is certain. If you land your headship, your school will benefit from the training preparation that you have already undergone and will still be entitled to receive. And so will you - if you groom your new colleagues for similar promotion.
* Secondary Headteachers: Job Factfile. This graph is NOT included on this database.