[iguy]

I think "selection" is the short answer. If you can, by any means, arrange to have a class of better-behaved more-interested children, then you can save money. First, you can pay the teachers less, because the job is much more pleasant.

Second, you can avoid hiring special-ed non-teachers, who (IIRC) are quite numerous in some public school systems (like 1/3 the staff?) but aren't usually counted in the teacher/pupil ratio (because their jobs are different).