They add minutes to the 4 days you are in session, most states have gone to tracking school attendance by the minutes in a seat. Which make sit possible for a district to complete the same amount of content in 1 less day. The 5th day is not off for all, coaches are still holding practice, admin still has meetings and so forth.
The deeper point you mention but don't say, is education just glorified babysitters? Isn't the model setup to allow both parents to work? It seems having both parents work correlates with the increase in divorce. Single parents work harder, make less money and stressed out trying to find places for their kids. What education is not the problem but the way we have set up society. Until we address the greater issue of destabilizing the home we will never fix education. If you want to fix the world, go home and love your family...as a great woman once said.