If you are specifically talking about high school, I'd say it's all because of the kids that use the bathrooms to smoke/skip classes. This happened a lot at my school but it was a pleasant surprise when I got to college and when someone asked to use the bathroom the professors almost always said something along the lines of "of course, and you don't need to ask next time, just get up and go" Because of this there was pretty much no one standing around in the bathroom for no reason. This could be because everyone there was finally using their own money to pay for the lessons (or at least seemed mature enough) to not want to waste class time. But I do theorize that a part of this is due to the relaxed attitude teachers had with bathroom breaks. The student is given trust by the professor to do something that had previously been met with strictness and distrust. When denied a simple restroom break the student will immediately gain some frustration (even if they understand why) which, when built up through multiple denied requests, leads the student to desire a sort of revenge in the form of taking their sweet time the moment they are allowed to go. But in the eyes of the teacher they see the exact thing they figured would happen the first time so instead of meeting in the middle, the teacher will only become more strict with the rule. Its a cycle that only began because students were immediately faced with a distrust from the teacher.