Note that it doesn't mention exactly what criteria they are using to determine what an "excellent teacher" is, or how individual teachers might have individual limits or preferences on class size, or even how it might vary from subject to subject or by particular group.
Also, speaking as a middle school educator, what I can do with class numbers depends so much on what the students come prepared to do. I currently teach 38 students in my "advanced" math course. These are the students that are taking high school Algebra I for in the 8th grade for high school credit. Managing 38 in that class is just fine. I have more to grade than my typical classes of 30 or less students, and giving individual feedback takes more time and effort on my part. Having more than 15 at times can be ridiculous. Generally, my Math Lab students despise math and do not come to me with very limited math goals and skills. In order to motivate, inspire and educate my Math Lab students attempts at adding beyond fifteen rarely works. As the year progresses, and student "buy-in" improves, I can add more, but I've never been able to justify having close to the 38 students that I have in my "advanced" math class.
I also teach a math support class, we call it the "Math Lab". The program at our school has been recognized by Michigan Association of School Boards as exemplary. This is mainly because we cap the class limit at 15 students and also have two dedicated classroom para-pro's (classroom adult aides) to assist me. In this class, management of the smaller number of 15 can be very difficult. I have to focus much of my attention on the student behaviors and motivating them before helping them with math. The kids taking the high school algebra in the 8th grade come to me as high-achievers that are ready and willing to put in an effort to learn. Although it's at a faster pace than the typical 8th grade course, and has much higher expectations, it's uncommon for students to carry a grade less than a "B" and management issues are minimal for me that hour.
I was never a teacher in an American public school, but as an EFL teacher, this really rings true. Also, props for teaching a math support class. I was one of those students myself and I honestly appreciated the teachers that did their best to help me out. On reflection though, I think that I wasn't really ready to learn math, or at least I couldn't understand why it was an important subject to learn. Of course, now I do, much to my chagrin. Especially as I am reviewing for the GRE. Anyway, group dynamics is something I'm really interested in and actually one of the graduate degrees I am considering is organizational psychology and more specifically conflict resolution. I have spoken to some people who think that conflict resolution is a hyper-specialization, but I've talked to others that see it as a fast-growing field with plenty of real-world application and demand. Either way, the field is growing. One thing that has always puzzled me about the public school system in the US is that class creation does seem to have some kind of logic behind it (at least in my experience) not nearly enough attention is directed at optimizing those groupings. Granted, there are limited configurations possible in most settings, but even so, group dynamics can make or break a class. In many ways, I feel like politicians should have experience with certain areas directly, since the decisions they make tend to affect those areas so greatly. The areas that come most immediately to mind are teaching, banking, economics and sociology. Of course, it is absurd to expect politicians to be masters of these things, but I think that greater compassion will lead to politics that better represent the will of the people and the only way I can think of to foster that is through direct experience. I think that this would lead to an understanding amongst politicians and the wider public that teaching a class of 38 collaborators interested in discovering something is a wildly different experience than paying intensive and individual attention to a class of 15 students poised to become adversaries at any given opportunity.Also, speaking as a middle school educator, what I can do with class numbers depends so much on what the students come prepared to do.