Well, the truth is they still can't.
In their report titled Getting Teacher Evaluation Right: A Background Paper for Policy Makers, Linda Darling Hammond, et al. write:
1. Value-Added Models of Teacher Effectiveness Are Highly Unstable. Teachers’ ratings differ substantially from class to class and from year to year, as well as from one test to the next..
2. Teachers’ Value-Added Ratings Are Significantly Affected by Differences in the Students Who Are Assigned to Them. Even when models try to control for prior achievement and student demographic variables, teachers are advantaged or disadvantaged based on the students they teach. In particular, teachers with large numbers of new English learners and others with special needs have been found to show lower gains than the same teachers when they are teaching other students.
3. Value-Added Ratings Cannot Disentangle the Many Influences on Student Progress. Many other home, school, and student factors influence student learning gains, and these matter more than the individual teacher in explaining changes in scores