As their case wound through the courts, plaintiffs in the Vergara v. California case returned over and over again to a central claim: poor and minority students are markedly more likely to have the lowest-quality teachers.
The Vergara plaintiffs hoped a court would strike down state laws that protect teachers' jobs, which they blamed for this inequity. Even when a state appellate court finally ruled against them, the plaintiffs noted the justices didn't dispute the evidence at the heart of their claim: a study by a Harvard economist, for instance, that found Latino students in the Los Angeles Unified School District were 68 percent more likely than their white peers to be taught by a "grossly ineffective" teacher.
So do poor kids truly have worse teachers? A newly-released national study suggests that finding from L.A. Unified may be an exception, not the rule.
A team of researchers studied the impact teachers had on their students' standardized test scores over five years in 26 school districts nationwide and found very little — if any — overall difference in the effectiveness of teachers in poor schools compared with their counterparts in rich schools.
"We found that both high and low-income students are … just as likely to have one of the very best teachers, just as likely to have one of the very worst teachers, and just as likely to have a teacher in the middle," said Eric Isenberg, a senior researcher at Mathematica Policy Research and the study's principal investigator.
The researchers did find some poor students would benefit substantially from higher-quality teaching in math, but only in three of the 26 districts they identified. They found black students have only marginally less effective teachers than their white peers in math, but in English language arts, their teachers are just as good.
There's no significant difference between the instruction white and Latino students are getting in either subject, Isenberg's team found.
The researchers also found only limited evidence of harm from the revolving door of teachers entering and exiting the profession or transferring between rich and poor schools. Vergara plaintiffs argued this churn — they called it the "dance of the lemons" — ends up leaving poor and minority kids saddled with the least effective teachers.
But Isenberg said, at least when it comes to a teachers' impact on students' test scores, most of the effects of this churn came out in the wash: teachers who left the profession tended to be less effective than average and newly-hired teachers tended to improve quickly. Overall, Isenberg's team concluded this churn did only slightly more harm to poor schools than it did to rich ones.
"I wouldn’t say we have evidence that 'lemons' are accumulating in high-poverty schools," Isenberg said.
Isenberg's team measured teacher effectiveness using basically the same statistical method behind the most controversial evidence in Vergara: a "value added model," which attempts to filter out factors beyond a teacher's control — a student's socioeconomic status, for example — to isolate what impact a teacher himself had on a student's scores.
The takeaway isn't necessarily that the research Vergara plaintiffs cited was wrong, but rather that what's true in one district — like in L.A. Unified, where Harvard economist Thomas Kane found an African-American student was 43 percent more likely to have a grossly ineffective teacher than a white student — isn't always true in another.
After all, the demographic profile of the 26 districts Isenberg's team studied is different from that of L.A. Unified — the student body in the districts Isenberg studied was 42 percent Latino; L.A. Unified is nearly three-quarters Latino.
In some of these 26 districts, the most effective teachers were somewhat more concentrated in lower-income schools. In others, effective teaching was more concentrated in richer buildings.
Isenberg said teacher quality ought to remain an important topic for researchers and lawmakers.
But as a matter of public policy, his team found that if poor students and rich students were taught by equally-effective teachers, the effect on the "achievement gap" in their test scores would be minimal.
"It does suggest," Isenberg said, "that a full-throated focus in all districts on equalizing access to effective teachers may not get the kind of results that putting our effort into a different set of reforms or different set of policies might be able to achieve."