If that is sustained over time, it becomes like compound interest, and what you see is populations of children constantly rising to higher and higher levels of achievement in later grades, regardless of where they started. Several states are discussing it. They also enacted legislation to start testing each child each year. Arizona is looking very seriously at testing each child each year in math and reading. Delaware is considering it, and Mississippi.
Every child needs to be tested every year.
If districts have been testing every year in one form or another, those results can be used to start getting diagnostic information before the state has completed testing with its annual regime. A state should pick four or five districts as pilots to work out a lot of the wrinkles while bringing on the statewide testing system. I recommend that none of the results be used as part of a formal evaluation until you get at least three years of data for each teacher. One problem I see is that some folks are talking about doing some rather simplistic things with value-added models. My approach is often criticized because of its complexity, but it's complex for a very important reason.