This got my attention. Toward the middle of the article, it points out that "The tradeoff is that students in the four-day system have to go to school 7.5 hours per day to get the same amount of instruction that the standard five-day, six-hour schedule provides. That means even 6-year-old first-graders have to be in class from 8:15 a.m. to 4:15 p.m. each day."
So, no, the students in these districts are not losing 20% of their instructional time. In fact, this may be a real boon to families with two parents working, or single parents, who have to come up with before-and-after-school care for their kids. Perhaps this move will be more succesful and widepread than the push for year-round school has been.