Evaluating teachers is very important. Evaluating teachers is also very difficult.
A recent proposal, that promises to make evaluating teachers simpler, proposes to link the assessment of teachers to their students' performance on standardized tests. If your students improve, you are a good teacher. Students' failure to improve casts doubt on the ability of the teacher.
For several reasons, this is a really terrible idea.
It is well-known that student performances in school are closely correlated with the economic condition of the parents. Poor children, on the whole, don't do as well in school. Are we really going to hold teachers responsible for the poverty of families in the neighborhood of the school?
These standardized tests, at least where I live, in Massachusetts, do a poor job in estimating the achievement of high school students. A significant number of the college students I teach, who have passed the state standardized tests, have serious difficulties understanding what they read. They just don't grasp ordinary expository prose. One problem is that many students have very limited vocabularies. In a recent class someone wanted to know what the word “superfluous” meant. Another student suggested that that it was synonymous with “superficial.” Both students were bright and interesting. But their high schools did a poor job of preparing them to read and communicate in college. The standardized test did not catch that deficit.
Should teachers be evaluated by tests that do not provide an accurate estimate of a student's intellectual skills?
Putting great weight on students test accomplishments in teacher evaluations is also unacceptable because it overlooks the most important information: the assessments by people in a position to know—other teachers, students and parents. Other teachers are one excellent source of information about a teacher's performance. Teachers work in the same school and pretty soon get a fairly reasonable idea of who is a good teacher and who is not.
When you evaluate teachers you must, of course, talk to the students. You want to ask them whether their teachers are well prepared, whether they are helpful to students, whether they are aware of student problems, whether their classes are interesting and engaging, and whether students believe that they learn something from this teacher. Is this teacher any kind of role model for students? Is the teacher fair but also understanding of special difficulties?
Finally the parents need to be consulted. They may very well have an idea of how the teacher influences their children or whether the child is particularly happy in this teacher's class.
Schools are communities of sorts. Everyone in the community needs to be consulted when it comes to evaluating teachers. The proposal to evaluate teachers by their students' test scores is one more top down bureaucratic project. Here the school is not seen as a community but rather as a hierarchical organization, much like the military, where the higher-ups have the power to foster the advancement of their subordinates or to end their career prematurely.
It stands to reason that schools that are run like military organizations are less likely to promote learning as an enjoyable and exciting activity. A flexible environment encourages learning and exploration. A rigid environment produces people who can follow rules but who will never be independent learners.
Basing teacher evaluations on student test scores is a bad idea.