I suspect the degree to which a headteacher could mentor staff in terms of quality improvement in teaching depends on the degree to which the institutional norm sees and appoints headteachers as:

a. Headteachers - who are effectively skilled planners and administrators.

b.Headteacher who are "head" "teachers" - promoted because they are seen as good teachers and able to support others.

Many years ago possibly the expectation was "b" but the nature of the role has changed very much into "a".

What is worth considering is what the formal qualifications for headship or school management involves. Where the qualification is an MBA there is seldom any reference to the heads role in directly mentoring other teachers. This is seen as a staff development role and often delegated to a specific member of the management who the supervises the process at department level.

Management courses for heads tend to focus on "monitoring" rather than "mentoring"...which are very different things. Whilst the head may ultimately manage the monitoring process I suspect few would actually see their role as directly mentoring staff.

 To enable in proper monitoring of implementation of any quality improvement programme using measurable parameters and enable in assessing the gap analysis between the present status and desired targets and to enable the preparation of Quality Improvement programme and in testing its robustness if there is already one.

Check list & Monitoring Parameters for Quality Improvement Programme in Education 

I suspect the degree to which a headteacher could mentor staff in terms of quality improvement in teaching depends on the degree to which the institutional norm sees and appoints headteachers as:

a. Headteachers - who are effectively skilled planners and administrators.

b.Headteacher who are "head" "teachers" - promoted because they are seen as good teachers and able to support others.

Many years ago possibly the expectation was "b" but the nature of the role has changed very much into "a".

What is worth considering is what the formal qualifications for headship or school management involves. Where the qualification is an MBA there is seldom any reference to the heads role in directly mentoring other teachers. This is seen as a staff development role and often delegated to a specific member of the management who the supervises the process at department level.

Management courses for heads tend to focus on "monitoring" rather than "mentoring"...which are very different things. Whilst the head may ultimately manage the monitoring process I suspect few would actually see their role as directly mentoring staff.

TAP schools (Teacher Advancement Program- System for Teacher and Student Advancement) are designed to provide routine professional development for teachers. In TAP schools, teachers participate in weekly professional development sessions that are aligned (well supposed to be aligned) with teacher evaluation data and routine informal observations made by mentors.  TAP schools hire additional staff (mentors, master teachers and administrators) to support teachers with their professional growth objectives.  The mentor and master teachers have a reduced course load so they can focus their attention to teacher evaluations and providing related professional development.  

I am providing an attachment and few links below that may be helpful.  The research in this area (TAP) demonstrates conflicting results, depending on your resources (that is often the case in educational research), however this research is relatively new and continues to develop.  It is certainly interesting and something to watch over the coming years.

http://www.niet.org/assets/PDFs/tap-research-summary.pdf

http://tapsystem.niet.org/action/action.taf?page=faq

To further complecate the issue is as Adrian points out why was the teacher promoted to "head" in the first place. I think it would be nieve not to take into account political influences, connections, and the possibility of a relationship. With all that has been said to this point the assumption that the title head teacher is in the best position to mentor is questionable at best.

Secondly, I would also think that the current form on monitering could be described as "grade checking". Meaning in the U.S. with our high stakes testing an arguement can be made that the montering of the teacher is the best "measured" by the test scores. This idea is also the arguement for teachers preformance being connected to teachers pay. One mesurement used in two very different contexts which is what some call efficency.

In the end, there are some assumptions in your question and they might need a little attention.

I hope this helps.

Douglas 

Use the key words in your research activities: leadership, school administrators. I have seen a lot of research in the US on this topic.  Quality leadership requires monitoring the teachers, and advising when needed on appropriate pedagogy. 

To be considered an external supervision actor, the actor must: have a mission clearly focused on control over or support to schools and teachers; Such a definition excludes, for instance, personnel working in teacher training institutions or curriculum development officers, who seldom visit schools. Staff who regularly visit schools, but only to collect data, are also excluded. Several actors can play a role in internal school supervision. They include the principal, the heads of department, the Parent Teacher Association, a school board. In many cases, though, their role is still very informal and sometimes not recognized. You need to decide to what extent they are important within your own country.

http://www.unesco.org/iiep/PDF/TR_Mods/SUP_Mod1.pdf

World over, highly effective teaching has been proven to improve student (or pupil) learning. However, the kind

of pedagogical practices teachers employ determine how successful students can learn, other factors

notwithstanding. In practice, however, many teachers often utilise professional practices that dampen the

chances of students to effectively learn. This kind of scenario may not be different with what is happening

amongst public secondary school teachers . Yet, effective school supervision could encourage teachers

to apply more robust and effective pedagogical practices that have been devised over the years. The

researchers delved into the effect of instructional supervision by school authorities on the pedagogical practices

of teachers in public secondary schools . The instigated by the persistent complaints from

different stakeholders about the deteriorating performance of students in many public secondary schools which they attribute to poor teaching.