There is plenty of strong evidence that we can use data to assess the impact of teachers on student outcomes. In a recent report, The Education Trust—West found that on average, students placed with the strongest teachers gained half a year more in English than students placed with the least effective teachers.

Differences of this magnitude are important for all students, even our highest achievers. But they can be devastating for low-income students and students of color who often enter school already behind. With a series of effective teachers, struggling students can quickly catch up to grade level. But one or two weak teachers can prevent them from reaching their academic potential.

Unfortunately, our research revealed that African-American, Latino and low-income students are far less likely to have access to the best teachers. Just as worrisome, high-need students can lose access to highly effective teachers as the result of quality-blind layoffs based solely on seniority. No wonder we have failed to close the achievement gap.

Given these high stakes, districts should be measuring the impact of every teacher on student outcomes. But test data is not enough. Research has shown that other measures, like classroom observations and parent and student surveys, can provide critical insights into how teachers are improving student performance. When combined with student achievement data, these “multiple measures” have the power to better assess teaching effectiveness.

If we are serious about helping all students achieve their full potential, we need to not just create these improved evaluations but use them to make real decisions. Districts should stop employing those teachers who don’t improve over time and ensure that students never lose access to great teachers. And they must provide the necessary incentives and conditions to attract the best teachers to the highest-need schools. The data from these evaluation systems can and should support teachers and school districts to make better decisions on behalf of students.