One of the factors affecting the school day is the length of class periods.  Those have always been pretty flexible in self-contained elementary schools.  But in high schools it's not quite the same.  For almost a hundred years there were Carnegie Units.  A Carnegie Unit is defined as 120 hours of class time over the course of a year at a secondary school for one hour, five days a week for 24 weeks per year. 

A Carnegie Unit defines the time necessary to cover the material necessary to acquire the information and knowledge that is needed.  Those who designed the concept knew what they were doing. (And I'm not a big fan of what the Carnegie folks have done since, but they had this concept right!) 

So what has been changed?  Well, high school students no longer have to stay in a class for one hour.  And the reduction in time is gradual and unnoticeable until one realizes that they've eviscerated the courses because now the teacher has 25% less time in which to cover material that used to be deemed important.  First they went to 55 minutes (they had done that by the 1960's), and now it's at 45 minutes and holding.

I don't think I need to explain what the effect of this lessening of time means.