Imagine you've just been hired for a creative, thinking-intensive job, and then you discover that your boss has 19 other people reporting to her as well. How do you think that's going to go? Conventional wisdom is that most most people are only able to manage half a dozen others effectively  — that's why sports teams are always about that size .
OK, now imagine that you've just enrolled in a school where students are expected to do most of their learning online via videos and exercises. There is a teacher, but she's there to help when you can't figure things out for yourself. Oh, and she has 19 other students . Why would you think that was going to go any better? Learning is the creative, thinking-intensive job; if most experienced managers can only handle half a dozen grown-ups before they start dropping balls, why should we believe that teachers will be able to handle more than half-a-dozen children before doing the same?
 Though many believe, wrongly, that they're able to handle more.
 Larger teams always break down into sub-units: forwards and backs in rugby, infield and outfield in baseball, etc.
 I'm being optimistic here: many classrooms have more than 20 students, and since many school boards' real motivation for moving education online is to save money, it's very like that student-teacher ratios will increase.