Stuffing five days of school into four days would be miserable and counterproductive for both teachers and students.  Imagine having the last class of the day--whatever time it started, it would be at the end of a nine-hour (or more) day.  Yeesh.  I'm exhausted thinking about it.  One meal would not be enough for such long days, and the extra weekend day would probably be spent just recovering from such an exhausting week.  More cost efficient or not, I vote no!

In exploring this topic, I would look to other countries to look at how much students go to school and how many days a week they attend school. Even in the US, various school districts have been toying with the idea of having a 4 day school week with longer hours, or having a 5 day week that lets out early one day a week for teacher conferences or planning. I think some arguments in favor of a five day week rather than a longer week is that school is only one aspect of life, and to be well rounded, students need weekend time to pursue other interests and learning opportunities. Another argument would be that students need time to recharge their "batteries" and rest before diving into another work week. 

In the interest of economics, one big part of the 5 day school week corresponds directly to the 5 day work week.  If the school week was to be shortened, what would working parents do with children on that 5th day?

You could also argue this by making comparisons of America's educational achievements to countries like Japan.  It is pretty common knowledge that Japanese students spend significantly MORE time in school than American children.  Perhaps you could find some figures to prove they are achieving more or have a more successful society as a result of their children spending more time in the classroom.

So... a five day school week as opposed to what?  As opposed to a 4 day week?  Or a 6 day week?  Here in the US a five day week is standard so I don't know what you would like it compared to.

Compared to a 6 day week, you can say that a 5 day week keeps students fresher and more motivated to learn.  Because they have two days off each week they do not get so bored of school.  They have some time off to relax and become motivated again to go to school.

Comparing it to a 4 day week, you can say that 4 days is too short.  You can say that 5 days gives teachers more time to teach and allows students to have enough time to learn.  You can say that 3 days off in every week is too much and that students forget what they have learned by the time the long weekend is over.