With two days of school left, why would a teacher show a movie? I believe part of the answer is in what economists call a moral hazard.
In the workplace, a supervisor can't observe all of the behaviors of her subordinates. Since working involves effort, it can be said working brings about disutility. If a teacher can show a movie instead of teaching at the time of year when the most disutility is likely to take place, the disutility is essentially zero. It's in the interest of the teacher to incur no cost by showing a movie. This is actually game theory.
Another reason why a teacher would show a movie is time inconsistency. When the benefits come now and the costs come later, it's easier to delay work. For a brilliant discussion with nice every-day examples, check out this blog. This blog goes by the name, beeminder.com.
Teaching is hard work especially at the end of the year. Few students are motivated to learn new material. Unfortunately, learning is a life-long task. Maybe economics will find a way to use commitment devices and change incentives so that both teachers and students can stay on task.