Students are required to always bring their device to my class. If they do not have it three times they will receive a discipline slip. Most of the other teachers use this policy. Some teachers, like PE or Science, tell students ahead of time if they should not bring their device to class. In this case, students must have the iPad LOCKED in their locker.
When in class, students are asked to have their iPad on full brightness. (This does run down battery some.) That way it is easy to see what students are doing. I think it is pretty obvious when a student is not on task, by the way he or she is clicking/looking at the screen.
Sometimes, I will take the device from students if it seems that they are too easily distracted during direct instruction time. If I do not want any students looking at their iPad, I will ask them to “flip” it so that it is screen side down. I have also had students sit on their hands, and fold their hands “in prayer.”
Students are often reminded that the school owns the device, and that we are just letting them borrow it!