A two-sigma effect is mind blowing. You can say that makes it more suspect but the magnitude of the claim must be emphasized, that’s just so frickin high of an effect size. (Personally I assume the claim is at least mostly true: how could full-time, good tutoring not be incredibly better?)
One does have to deal with the social aspects--not that all scoial aspect of secondary school are positive. But it seems obvious that 1:1 tutoring is generally positive beyond the cost. Certainly I got a lot out of independent study classes in grad school (and to a lesser degree undergrad).
