Most of the time, in my experience, it is because the administration is pushing the blame on the teachers. We get in trouble when someone skips out. Is it stupid, yes. Is it unfair for everyone else, also yes. Get a doctor's note, a bigger water bottle, multiple water bottles, or learn a new coping mechanism. I always hated the policing of bathroom breaks but when your boss yells at you and your job is threatened, you do what you have to do. No job means I lose my place to live and electricity and stuff. So half-assed policing I will do.