One suspects that more experienced teachers get handed easier to teach students. Not always of course, but on average. One mechanism is that more experienced teachers move to districts with higher socioeconomic status teachers, or move to “nicer” schools within a district. Demetra Kalogrides, Susanna Loeb, and Tara Béteille look at assignments within schools in Miami-Dade County (the fourth largest U.S. school district.). Perhaps not surprisingly, the more experience a teacher has the higher her students’ test scores the previous year.
Here’s the authors’ picture.
Prior year math score versus teacher experience
You can see that more experienced teachers are handed students who had relatively better scores the year before. To help understand if this is a big effect or just small one, it helps to know that a year of learning corresponds roughly to 0.6 on the vertical scale. So for middle/high school teachers, the effect is quite small for the first eight or nine years of experience. At the elementary school level, a teacher with five years of experience gets handed students who are almost a month ahead of the students given to a new teacher.
The research on the effect of experience on student learning is clear that teachers don’t do as well their first two or three years. After about five years it appears that the experience/results profile is flat, although the issue isn’t absolutely settled. This suggests to me–but doesn’t prove–that assignments largely reflect the clout of more experienced teachers, rather than principals trying to match teachers and students in the most effective way.