What is the ideal class size? If you listen to popular rhetoric, we are told that 25 to 35 is really, really bad, and 15 is really, really good.
While I agree that 15 is better, that's kind of like saying driving 95 mph is better than 125. Anyone who has tried to interact with 15 children know the activity is still that of sheep-dogging and projecting content.
Now ask yourself, if you weren't constrained by budget or logistics or even common-sense, what would the ideal class size really be?
Really, class size should be about 5. This allows mini peer-to-peer conversations, while allowing both common presentations and shared one-on-one interacting that is necessary. If you include the coach, it is also an even number. 5 is most often better than either 4 or 6.
(With this ideal class size comes a redefinition of class, from "class being entire community all day" to class being focused learning.)
From the point of view of the education industry, 15 is a reasonable "best" goal - a calculated stretch goal for more funding, without actually meeting the needs of the students. Once you get past their framing, the real number is much different.
Author's note: when I led hiking and canoeing trips in Maine, we did have a ratio of instructor to student of 1:5, with two instructors for ten kids. It is possible, even in a more institutional setting.