Nobody objects to teachers being evaluated on their effectiveness. Using gains on standardized tests is a bad way to do it.

Need for better evaluation system
Published in USA Today, Feb. 7, 2012
The article "States weaken tenure rights for teachers" emphasizes the importance of evaluating teacher effectiveness. A major problem is that these evaluations are often based on students' gains on standardized tests, called "value-added" measures.
A number of studies have shown that value-added measures are very unstable: Teachers' ratings based on previous years are weak predictors of test scores at the end of a year with new students. A teacher who succeeds in boosting scores with one group will not necessarily succeed with others. Different tests can result in different scores for the same teacher.
Value-added evaluations also ignore the huge impact of factors beyond the teachers' control. Finally, there are ways of pumping up test scores without student learning, including teaching test-taking strategies and making sure weak students don't take the test.
Nobody objects to teachers being evaluated on their effectiveness. Using gains on standardized tests is a bad way to do it.
Stephen Krashen, professor emeritus; University of Southern California; Los Angeles