The story here is as straightforward as it is disheartening: on average, new teachers hold their students' learning back. Of course, we can't expect novices to do as well as more experienced teachers. But surely it's not too much to ask that teacher training equips new teachers to help their students stay on track.
If that weren't bad enough, here's what Harvard University's Strategic Data Project (SDP) found when they looked at placement patterns of teachers in the LA Unified School District:
SDP Human Capital Diagnostic Los Angeles Unified School District (November 2012)
On average, LAUSD novices are assigned students who are .2 standard deviations -- or six months -- below the average in math achievement. SDP has found the same sort of placement pattern in other big districts it has studied. Given what we know of the effectiveness of first-year teachers, this is a recipe for systematically widening the achievement gap.
Clearly, districts are going to tackle this issue head-on and think of ways to incent their more effective teachers (who will tend to be more experienced) to teach struggling students. And teacher educators are going to have to further refine their programs to deliver more teachers who are classroom-ready.