This is the third part in a series I’ve been writing this week about the report, How the World’s Best-Performing School Systems Come Out on Top, which is an analysis of the world’s school systems to find out why some schools succeed and others do not.
Today, my focus is on the section of the report that analyzes the widely supported reform strategy of class size reduction. It turns out that reducing class sizes does not significantly improve student outcomes. Instead, it significantly worsens student outcomes because it reduces teacher quality.
Here’s how reducing class sizes also reduces teacher quality:
“Class size reduction, facilitated by lower student-to-teacher ratios, has probably been the most widely supported and most extensively funded policy aimed at improving schools.” Over the past five years every country in the OECD except for one has increased the number of its teachers relative to the number of its students.
Yet the available evidence suggests that, except at the very early grades, class size reduction does not have much impact on student outcomes. Of 112 studies which looked at the impact of the reduction in class sizes on student outcomes, only 9 found any positive relationship. 103 found either no significant relationship, or a significant negative relationship. Even when a significant relationship was found, the effect was not substantial. More importantly, every single one of the studies showed that within the range of class sizes typical in OECD countries, “variations in teacher quality completely dominate any effect of reduced class size.” Moreover reducing class sizes had significant resource implications: smaller classes meant that the school systems needed more teachers, which in turn meant that, with the same level of funding, they had less money per teacher. It also meant that because the school system requires more teachers to achieve smaller class sizes it could become less selective about who could be a teacher.
Further evidence that improving teacher quality is more critical than reducing class sizes:
The available evidence suggests that the main driver of the variation in student learning at school is the quality of the teachers. Ten years ago, seminal research based on data from Tennessee showed that if two average eight-year-old students were given different teachers—one of them a high performer, the other a low performer—their performance diverge by more than 50 percentile points within three years.
By way of comparison, the evidence shows that reducing class sizes from 23 to 15 students improves the performance of an average student by eight percentile points at best.
Because teacher quality affects student outcomes much more than class size, policy makers in top-performing school systems invest resources accordingly.