The consequences of teacher churn were apparent to Esperanza Vazquez, a mother of two from New York City.

I had an experience with my son where he had a new teacher every week in math,” she told Chalkbeat recently. “That doesn’t help students.”

Now new research backs up Vazquez’s experience, documenting for perhaps the first time the steep consequences for students after teachers leave a classroom in middle of the school year.

The finding comes in a trio of new studies focusing on North Carolina. Together, they suggest that ill effects of teacher turnover identified in previous research may be driven largely by midyear departures; that those consequences extend even to students in the same grade whose teachers stay on; and that midyear turnover may be more common than previously thought, especially in schools serving more students of color and those from low-income families.

“While it is possible for turnover to be beneficial for school systems, an extensive body of research points to the ways that teacher turnover disrupts … the continuity of a child’s learning experiences, particularly in underserved schools,” write researchers Gary Henry of Vanderbilt and Christopher Redding of the University of Florida in one of the papers.

Henry and Redding’s three studies — two of which were published earlier this year in peer-reviewed journals, with the other is set to be published in coming weeks — home in on the rarely studied phenomenon of midyear teacher turnover.

Using recent data from North Carolina, two of the papers focus on the prevalence of the phenomenon. Annually 4.6 percent of teachers in the state departed midyear; among teachers in their first three years the rate jumped to 6 percent. The number was higher in schools deemed “underserved,” meaning they had more students of color and students in poverty, as well as lower test scores and fewer resources. Turnover was lower when principals were rated as more effective by teachers. It was higher among teachers who were less effective, those eligible for retirement benefits, and high school and middle school teachers.

Roughly a quarter of all teacher turnover in the state occurred in the middle of the school year.

The third study uses data from 2008 to 2014 to examine the consequences of midyear teacher attrition on elementary and middle school students’ test scores. In both math and English, students saw drops in learning as a result, controlling for a number of other factors. The decline in math scores was nearly as large as the difference in performance between an average teacher and an excellent one — a difference that has motivated dramatic policy changes in many places.

Impacts were smaller in English and in middle school, but also consistently negative. Students in the same grade level, but not class, of teachers were also harmed, but again less so.

The negative results are consistent with research on the effects of hiring teachers after the school year starts, in some ways a mirror image of the phenomenon.

The paper suggests three things that might explain the results: disruption in classrooms where teachers leave, instability in a school where teachers are exiting midyear, and less effective teachers replacing those who depart. The study suggests the first two theories seem to be clearly at play, since it was relatively ineffective teachers who were particularly likely to leave.

“When multiple teachers exit a school during the year, it can become increasingly difficult for teachers to maintain a work environment with a high degree of collaboration,” the researchers say.

The study did reach some surprising results: Students of color, students in poverty and students with lower prior test scores, generally did not suffer more as a result of midyear turnover; if anything, they suffered less in English. It may be that their schools were better prepared for midyear exits since they happen more frequently; it could also be that those students were simply “not well served by the teacher who departed,” the paper hypothesizes.

Another counterintuitive result: Unlike midyear turnover, departure of teachers at the end of the school year did not lead to declines in student learning, and even led to small benefits in some cases. That’s surprising in light of past research — and conventional wisdom — suggesting that teacher turnover harms students. (Prior studies generally have not distinguished between midyear and end-of-year turnover.)

The latest research does come with a key caveat: Test scores might be lower in classes where teachers leave midyear for other reasons — perhaps a particularly disruptive class causes both a teacher to quit and students to learn less in school. The authors attempt to account for this by comparing how the same student did in years when their teacher does not turnover.

The studies also look at just a single state, so it’s unclear whether the results would look similar elsewhere.

The researchers point out that some churn is inevitable, even healthy. “Many of the personal factors driving within-year teacher turnover are unlikely to be amenable to change: a teacher takes time in the middle of the school year for parental leave; a veteran teacher retires midyear; a beginning teacher leaves a few months into the school year after realizing teaching is a poor occupational fit,” write Henry and Redding. Indeed, female teachers between the ages of 26 and 40 years old were particularly likely to exit mid-year, indicating that parental leave plays a significant role in the results.

But the studies collectively conclude that students could benefit from combating midyear departures — although the best way to do that is not clear.

In Detroit, some schools have adopted a crude — and some would say cruel — approach, imposing financial penalties for teachers who left midyear. Studies focusing on turnover in general have found that higher pay, better working conditions, and more effective principals can make a difference.

At the same, time Henry and Redding argue that policymakers ought to make extensive efforts to avoid midyear teacher turnover when possible. For instance, they point out that information from teacher evaluation systems, including “value-added” test scores measures, aren’t always available until after the school year has begun. Finalizing those results before classes are underway could decrease midyear exits, they speculate.

“All measures of teachers’ performance, including their value-added scores, should be provided during the summer to allow teachers and administrators to attend to employment decisions without disrupting classes that have already begun,” the researchers conclude.