They are 30 minutes longer than my high school classes were, but we had 7+ different classes per day. I'm pretty sure our day lasted till 3:30 or 4pm. We had 5 minutes to get to the next class, and on a big campus, that could result in some pretty good cardio workouts. So maybe inadvertently we were getting those stretches recommended by the author.
(Note: we have a block schedule; not all classes meet each day)
So enter block scheduling: the idea is to lengthen the classes to 90 minutes or so and alternate days when they meet. You still lose 10 minutes of each class, but that's 8/9 of your nominal instruction time, not 5/6. And teachers get a bit more flexibility in terms of what things can be done with a class. The drawback is that if you lecture for the full 80 minutes, the outcome is typically not so great: it takes an exceptional lecturer, or an exceptional student, or for some sorts of material both, to pull off someone still being able to absorb information in a lecture that's longer than an hour.