The fact is that the overwhelming majority of schools do not randomly assign students to teachers. This policy caused little concern in the past because teachers were not evaluated on the basis of the value they added to their students' knowledge and skills. But as Jesse Rothstein wrote: "Non-random assignment of students to teachers can bias value added estimates of teachers' causal effects" ("Student Sorting and Bias in Value Added Estimation: Selection on Observables and Unobservables").
States can't have it both ways. If they are going to adopt the value-added model as the centerpiece of their teacher evaluation systems, then it behooves them to use random assignment of students. By failing to do so, they seriously call into question the inferences made about the effectiveness of individual teachers. Confidence is indispensable when the stakes are so high.
(KC Supe) Covington went on to say that his staff had identified the “best” teachers in the district and would be giving them additional students. This was less than a week before school was scheduled to begin. The day after this announcement, teachers in the early grades received their class lists. Some first grade teachers were assigned 37 students per class, and some kindergarten teachers had 25-30 – compared to other teachers in the same schools, who had twenty students per class. Interestingly, some of these larger classes were staffed with brand new Teach for America recruits.
Then on August 19, Covington hosted a breakfast for eight elementary classroom teachers from about six schools out of 23, in grades 3-5th, whom he identified as “the best in the district.” He did not explain how he determined that they were the best. He told them that if they were willing to take 6 to 8 additional students, he would give each of them them $10,000. This would mean they would have class sizes in the mid to upper thirties.
For the first year ever, principals were not allowed to assign teachers or kids to classes within their own buildings. Covington’s staff did all of that. They decided who would teach what grade level and which kids would be assigned to each teacher. Before, this has ALWAYS been handled by each principal for his/her school.