We are trained in the education field to structure our lessons around the ‘fact’ that a student can only pay attention for about 20 minutes. This is part of the pedagogy of most education programs (and God forbid you argue against it). And then there’s the common structure of school itself — abrupt and significant breaks in activity every 60 to 90 minutes. And not small shifts either. A kid goes from the slow act of reading literature to the structured process of mathematical formula to sudden physical activity, usually with about 5 minutes between each.
If kids can’t focus it’s not because they are born that way; it’s because we made them so.