My initial observation is that a lot of students take out their cell phones as soon as they walk out of a classroom and I want to know how many students actually do take out their cell phones before they even leave the building. My hypothesis is that at least 50% of students do this.
A good test for this would be to set up a camera both right outside of the classroom door and in front of all of the outside doors. You would need to record the end of several classes. Afterwards you would need to add up both the total number of students and the number of students on their phones. Once you have those numbers you would need to divide the number of students on their phones by the total in order to get a percent. If that percentage is equal to or greater than 50%, then the test would support the hypothesis. You would need to continue testing in order to know for sure whether or not the percentage is always the same.
If the percentage is less than 50%, you would need to revise your hypothesis. In this case it would be by lowering the percentage of students that you think pull out their cell phones after class. You would then need to redo your test to see if the results are consistent with your new hypothesis.
If they are, you would have to continue testing. If they aren’t, you would once again need to revise your hypothesis.