This post originally appeared at the Fordham Institute’s Flypaper blog.

For decades, lowering class size has been touted as a strategy for improving student learning, despite loads of research asserting that it is not an effective solution. The Fordham Institute’s new study, Right-sizing the Classroom: Making the Most of Great Teachers, turns this idea on its head with a simulation of what happens if good teachers are actually assigned more students. The result: increased student learning.

This study uses teacher and student data from North Carolina and simulates what would happen if teachers with high value-added — those who are advancing student learning at greater rates than predicted — were assigned between six and twelve additional students (and if less effective teachers’ classrooms were proportionally reduced by this many students). The findings are promising: Adding six students to effective eighth-grade teachers’ classrooms could produce gains equivalent to an extra two weeks of school. And because more students are exposed to effective teaching and fewer are subjected to less effective instruction, schools experience improved student learning overall.

Findings like these should ideally put an end to the notion that blanket reductions in class size are a solution to anything. And it should launch a conversation about how to promote staffing and compensation models that vary how students are assigned to teachers based on their effectiveness.

However, the promising findings from the study could overshadow another important result: increasing class size for effective teachers does nothing in this simulation to help low-income students gain more access to effective teachers. This is likely because too many of our highest-need schools with high concentrations of low-income students don’t have enough effective teachers in the first place; shifting students around within a building doesn’t address this.

So while the premise suggested through this study is promising and worth exploring in actual practice, the strategy can’t just be about getting more students in a classroom with effective teachers. It also must be about getting more effective teachers into the buildings where they are most needed. This can happen in unconventional ways — for instance, Public Impact’s Opportunity Culture work explores how to use technology to extend the reach of top teachers. But it also needs to happen by addressing school culture and school conditions, such as the quality of leadership and the opportunity for teachers to work collaboratively and develop professionally. These are factors that keep great teachers from going to (and staying in) high-need schools.

By exploring flexibility with class sizes, we may ensure that more students in certain schools have access to great teachers, but until we address the bigger issue of which teachers are teaching where, we will never reach the goal of equal access to effective teachers for all students.