Rating Teachers by Test Scores

A current issue in education is finding a method of evaluating teachers.  The big push is to reward good teachers and get rid of the poor ones.  One of the suggested methods of evaluating teachers is through their students' standardized test results.  Their performance on these tests is to determine a teacher's worth.  This is a terrible idea.  If this evaluation method becomes widespread and standard procedure in education, there will be negative results.  The teachers are going to focus all of their efforts on the standardized tests.  They will be more concerned with saving their job rather than teaching effectively.