I like high school students to control when and if they may attend to business such as calling home, rest room, locker etc. It places the responsibility with them, where it belongs. However, I want some control over how often they leave, so at the beginning of the term, I give each student a set of personalized pre-printed passes. I decide how many times I think it is reasonable for them to leave, and give them that number of passes. To use a pass, students fill it out with date, destination, and time, and leave it on my desk and then leave. Meanwhile, I have not been interrupted from explaining an assignment, helping a student or whatever. The student has not wasted time waiting for my attention; I have a record of where they are, and they, not I, decide if they really need to leave. When students have used all of their passes, they may not leave. I make that clear from the beginning, and warn them to plan ahead, and consider saving at least one pass for emergencies.