of the world's best-performing school systems by US management consultancy McKinsey found that improving "instructional quality" was the most significant factor in raising student outcomes. So DTL is a weighty role indeed, with many functions and required skills, which can be summarised as:
Helping heads of department
This is likely to involve being part of their appraisal process, agreeing departmental development objectives, discussing training needs, scrutinising exam performance and ensuring that appropriate quality assurance is undertaken through work sampling, lesson observations and minutes of departmental meetings. The ability to diplomatically chair meetings with heads of department is a dark but much-desired art.
At a strategic level, this involves identifying where the staff body most needs to improve. These issues might emerge through inspection, changing priorities or a collation of "areas for development" gleaned from lesson observations. Such priorities might also be identified by asking teachers and students how they can most effectively be helped. It is important to win the hearts and minds of the staff when such initiatives are imposed from above - sometimes by the government - and they must be galvanised to comply in a way that doesn't compromise their values as teachers. The ability to impart some positive spin alongside a sympathetic smile is an advantage.
Getting the right person for the job
A key responsibility is to advise the headteacher on staff appointments, based on a rigorous testing of a candidate's classroom skills, a probing interview and feedback given by a student panel.
What sort of person in a school is best qualified to tackle such a central job? And what can an aspiring DTL do to position themselves for this stimulating and rewarding role?
Most commonly they will have come up through a head of department role, and so have had the chance to manage a team. They will have a proven ability to make sound judgements about the quality of the classroom experience and offer judicious advice for improving it. They may have participated in, or led, a peer-observation programme or undertaken training in lesson observation. They will be familiar with trends in teaching and learning and able to think critically about such issues.
Offering constructive and supportive feedback to teachers on their performance is a key skill, and although there is a certain amount of natural empathy and diplomacy that goes into this, significant improvements can also be made through training and practice. The most influential factor in my own development as an observer of lessons was my training as an inspector, which offered the chance to debate the merits of numerous pre-recorded lessons according to agreed criteria. Over time my fellow trainees and I achieved an increasing level of consensus about what constituted an excellent lesson.
Anyone with an ambition to be a DTL should find a way of joining this formative debate, whether through formal training, participating in learning walks or setting up a group to debate the merits of commercially available video lessons.
Perhaps the most crucial prerequisite for a DTL, however, is that they should themselves be excellent practitioners with a love of the classroom and the teaching process, and with a clear vision about what constitutes great teaching that they seek to share with others.
Alistair McConville is director of teaching and learning at Bedales School in Hampshire