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.