These studies rarely see the light of day in the American discussion. I have added a tab on the home page that includes the best of them.
My analysis of these studies was and remains that class size *can be* meaningful in two cases. The first is in early elementary grades. Here very small classes can make a difference if the teachers involved are capable of making the most of small class size education. Not all teachers teach small classes well.
The second case are in classes at any level where the critical mass of students have exceptional needs either personally or due to the complexity of the subject matter. Certain art classes fall into this category. Remedial classes fall into this category.
I found no study that could claim that universally small classes yielded consistently superior students or learning experiences. Every study found that the anecdotal opinions of teachers were that they and the students were far better off in small classes yet no such thing could be discerned in comparing a small class to a larger class. The union propaganda never mentions this even though the STAR study and others make this point clearly. Disingenuously the union message is that class sizes are very important without saying that this assertion is based on teacher preference and not cost or learning effectiveness.
A new tsunami of evidence is washing ashore thanks in large part as a reaction to the propaganda wars between Diane Ravitch and her consumer groups and the political forces trying to reform eternally troubled schools.
Let's take a look;
Brookings recently released this report that concludes:
Because the pool of credible studies is small and the individual studies differ in the setting, method, grades, and magnitude of class size variation that is studied, conclusions have to be tentative. But it appears that very large class-size reductions, on the order of magnitude of 7-10 fewer students per class, can have significant long-term effects on student achievement and other meaningful outcomes. These effects seem to be largest when introduced in the earliest grades, and for students from less advantaged family backgrounds.Almost immediately the Brookings study was attacked by an organization called NAEP. Diane Whitmore Schazenbach asserts that the Brookings study...
When school finances are limited, the cost-benefit test any educational policy must pass is not “Does this policy have any positive effect?” but rather “Is this policy the most productive use of these educational dollars?” Assuming even the largest class-size effects, such as the STAR results, class-size mandates must still be considered in the context of alternative uses of tax dollars for education. There is no research from the U.S. that directly compares CSR to specific alternative investments, but one careful analysis of several educational interventions found CSR to be the least cost effective of those studied.
conclusion is based on a misleading review of the CSR research literature. The report puts too much emphasis on studies that are of poor quality or that do not focus on settings that are particularly relevant to the debate on class-size policy in the United States. It argues that class-size reduction is less cost-effective than other reform policies, but it bases this contention on an incomplete accounting of the benefits of smaller classes and an uncritical, unexamined list of alternative policies. The report’s estimates of the potential cost savings are flawed as, in reality, schools cannot structurally reduce class size by only one student. Well-documented and long-term non-academic gains from CSR are not addressed. Likewise, the recommendation for releasing the ―least effective‖ teachers assumes a valid way of making such determinations is available.Her criticism is hailed by Ravitch, Haimson, and others as if it is proof-positive that smmall class sizes are a panacea to the educational chaos. In fact, the criticism is both shrill and silly. Chingos of Brookings calls her out in a rebuttal;
The California and Florida evaluations certainly have significant limitations, but in my view they provide preliminary evidence that large-scale policies are unlikely to produce benefits as large as those found in Tennessee. But applying Schanzenbach’s standard for studies leaves us with no studies of these kinds of large-scale policies. It seems awfully hard to make a case for large-scale CSR policies if we know essentially nothing about their effectiveness.
Even more important than the effectiveness of a policy is its cost effectiveness. As Russ Whitehurst and I argue in our paper, the right question to ask about any policy is not whether it has any effect at all, but whether it is the most effective use of limited resources. Unfortunately, there is little rigorous evidence on the relative cost-effectiveness of various education policies. There is a clear need for such evidence, but in the meantime it seems unwise for policymakers to mandate widespread adoption of a costly policy with uncertain benefits.
CSR may well be cost-effective in some circumstances, especially if it is implemented in a targeted way. For example, a district may find it sensible to provide small classes for its most disadvantaged students or its newest teachers. But CSR mandates take exactly the opposite approach in that they apply across-the-board and take away schools’ autonomy to decide whether reducing class size is the best use of limited resources.
But Chingos gives the STAR study a pass as I do on its conclusions. Not all critics are so kind. It is not a foregone conclusion that classroom size reduction (CSR) truly makes a difference even when its cost is not part of the calculation.
Andreas Schleicher as profiled in the Atlantic reinforces Chingas' arguments.
He concluded that the best school systems became great after undergoing a series of crucial changes. They made their teacher-training schools much more rigorous and selective; they put developing high-quality principals and teachers above efforts like reducing class size or equipping sports teams; and once they had these well-trained professionals in place, they found ways to hold the teachers accountable for results while allowing creativity in their methods. Notably, in every case, these school systems devoted equal or more resources to the schools with the poorest kids.
And the Educational Writers Association just published some interesting studies about teacher effectiveness.
Are teachers the most important factor affecting student achievement?
This has become the default first sentence of many speeches and reports on teacher quality. Recently, it’s become common to clarify that teachers are the most important “school-based” factor in learning—a critical qualification, given that factors external to schools exert more influence overall on student achievement than any factors inside the school.
A famous 1966 study by James Coleman found that background characteristics such as race, parental achievement levels, and family income swamped most other factors studied as determinants of student test scores. Decades of research have confirmed this study’s general findings, with a 1999 paper estimating that 60 percent of variation in student achievement was attributable to such background characteristics. 
Researchers have been unable to link a significant share of the variation in student achievement—as much as 25 percent—to any particular input. Of the remaining share, attributable to what happens within school, researchers have linked most of that variation to teachers.
It is difficult to cite an exact figure on what percent of the variation in achievement observed is attributable to differences in teacher effectiveness. Three economists in 1998 estimated that at least 7.5 percent of the variation in student achievement resulted directly from teacher quality and added that the actual number could be as high as 20 percent.
Researchers have found that school-based factors, including teaching, are more influential in math than in reading. A 1999 paper puts all in-school factors, including school-, teacher-, and class-level factors, at approximately 21 percent of the variation in 10th grade mathematics achievement. It further estimated that 8.5 percent was directly due to teacher effectiveness.
Some researchers warn that other important factors that potentially affect achievement— such as the effect of principals and other administrators, and the interaction of teachers with the curriculum—have not been as carefully studied as teacher quality.
It can be said:
Research has shown that the variation in student achievement is predominantly a product of individual and family background characteristics. Of the school factors that have been isolated for study, teachers are probably the most important determinants of how students will perform on standardized tests.
The myth of small class size will not go away for lots of reasons. But as we watch our state and national prosperity continue to evaporate we would be doing ourselves a favor to opening our eyes to the facts.