The abilities of teachers to teach vary a great deal, but the abilities of students to learn vary to an even much greater extent. If a student fails to learn, is it necessarily the fault of the teacher? It’s nearly impossible to measure how a teacher has improved the ability of any one student, let alone a classroom full of students. @ucbalumnus also pointed out that using students’ test results to measure teachers’ performance incentivizes the practice of “teaching to the test”.