[zozbot234]

> focus most of your resource on the lowest performers as they are the easiest to improve

That depends heavily on the pedagogical approach. There are approaches that are quite effective in bringing low performers up to near-par (so-called "direct instruction", in a broad sense) but teacher actively hate them because they're viewed as "demeaning" the profession, and ed schools don't teach them. Special Ed teachers actually get extensive instruction in these approaches, but obviously we cannot and should not treat every low performer as Special Ed.

> Proper student:teacher ratios

What's "proper"? Teacher-centered and direct approaches cope quite well with greater class sizes, but again they're unpopular among teachers.

[bsder]

> Teacher-centered and direct approaches cope quite well with greater class sizes, but again they're unpopular among teachers.

Cite. All research I have seen completely contradicts this.

The limitation with larger class sizes is not "knowledge transfer"; it's "classroom management" aka dealing with a student causing an interruption for some reason (bathroom, injury, sickness, etc.).

I use the Gates Foundation as my primary citations because they are easily findable on the web and simply match all of the other findings. You max out at about 15 students per 2 teachers because one of the teachers can handle the inevitable disruption while the other teacher can continue teaching. The more students you add on top of the roughly 15, the more likely you wind up with 2 interruptions which stops the class cold irrespective of teaching technique.

And, as I have stated, most of the research focuses on elementary to middle levels. High school requires teacher specialization which confounds a lot of the data.

[zozbot234]

How Does Class Size Vary Around the World? (OECD, 2012) shows that there are countries such as Japan and South Korea with some of the largest average class sizes, and excellent educational achievement. Clearly it's far from the only or even the most important factor. The United Kingdom is one country that skews the other way, though there are countries like Estonia with similarly small classes that seem to do a lot better.

[Nevermark]

I know nothing about those countries ratios.

But on the topic, I suspect the best time to have high teacher ratios is in the earliest years. Help children learn to manage themselves, focus, follow through, etc. Then work up to large class sizes in the upper years.