Except that now you are ranking, by introducing the notion relative to everybody else.
Why should test scores improve year over year? If everyone is getting a 2/6 then yes, but students in the same grade are not going to get smarter every year. There is the major flaw in using student tests to measure teachers IMO. You are going to get a bell curve around 4/6 from the test results for majority of the teachers. When you don't, it is really hard to tell whether the reason is the teaching, the curriculum, the students or the test and its grading.
Why should test scores improve year over year? If everyone is getting a 2/6 then yes, but students in the same grade are not going to get smarter every year. There is the major flaw in using student tests to measure teachers IMO. You are going to get a bell curve around 4/6 from the test results for majority of the teachers. When you don't, it is really hard to tell whether the reason is the teaching, the curriculum, the students or the test and its grading.