Of course teachers, like any professional, should be judged on their product. A teacher is trained to teach. If a teacher consistently fails to teach their students, they should be judged a failure for sure! If the school that trained the teacher consistently turns out failing teachers, that institution should be judged. If a school continually hires ill-trained teachers, they too should be judged. The exception comes when the professional is not in possession or control of the implements of their profession.
Teachers who work for learning institutions, unlike professionals with private practices, are not in control of all the aspect that make effective teaching and learning possible. They work for institutions that control, who their students will be, how many there will be per classroom, the length, dates and times of classes and, depending on the school, a large part of the curriculum. They are literally made responsible teaching, without being given the power to control the teaching process. Judgment of these teachers must include an evaluation of the level of responsibility and control they have over the process. This may result in a judgment of the institution/school, as well.
Take for example, a well-trained teacher, who is put in a situation of overcrowded classrooms, inadequate teaching tools and supplies, inadequate working environment, no control over the curriculum, no support from colleagues and administrators, etc? This teacher, no matter how well trained, will find it difficult to teach in this environment and may fail. This teacher is faced with moving on in search of a better school, where they can adequately practice their profession, or staying and being judge a failure because the kids aren’t learning.
Yes, like any skill or profession, teachers should be judged on their performance, i.e. are their students learning. Where teachers have control over their implements of work, (tools, supplies, environment, curriculum, etc.) they can and should be judge as you would any profession. If it were a doctor, you would want to know if his patients are well cared for. If it were a lawyer, you would want to know if their clients win their cases and are receiving good legal advice, etc.
However, when it comes to judging teachers who have little or no control over the necessary implements of their profession, they must be judge differently. Judgment of these teachers must take into account their control over the implements of the profession. In fact greater merit should go to those teachers who work under inadequate circumstances where the controls of the working circumstances are in the hands of others, i.e. Administrators.
These teachers must be judged, but so too, most the facilities at which they teach, the administrators they teach under and anyone else who is not the professional/teacher, that controls the implements of the profession/teaching.