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.