The Teacher and the System
The 'good teacher' inside of individuals is a potential. The teaching system that socializes people into teachers, or socializes the entire society to value teaching, education and the teaching profession is the extant reality. The emphasis on evaluation must address both the teacher and the system.
Part of the answer to a good educational system is the teacher, but let's not exaggerate the personal responsibility. There is an institution of education which dovetails with several other institutions. If the connections between the institutions, and within each institution, are too far apart, too differentiated, and not integrated enough, then individuals stick out as problems when the institution does not produce achievement. There are many stakeholders who are responsible from parents, to community leaders and politicians, to universities and especially graduate schools, to professional organizations, to unions, and ultimately to funding decisions made at the political level - the primary system of the social system!
Blaming the teacher is only appropriate in certain cases, most teachers having qualified through the university system by acquiring credentials. Supervision of teachers is perhaps an especially important key area because individuals have to be socialized on the job to perform; resources and training for the specific curricula and student population - the school or community - must be present and must be good for results to be forthcoming.
Look to the organizational dynamics of schools and schooling as a system, an institution. For example, you can teach a curriculum as best as you possibly can but if the students are closed, will not pay attention, or do the reading, education does not work. Getting under the surface problems to motivations, values and especially norms in the community, historically, and socially is very important to creating change in the teaching-learning dynamics in the classroom.
Some teachers can bear the burden of the entire social force and come through looking good with high rates of student approval and achievement, but these stars are few. More importantly is the issue of motivation, norms and values regarding learning, lifelong learning, not just for the degree, or for the job. This enters into questions of the spirit of a people, their interests, and their appreciation of knowledge!