Where’s your hall pass?
I don’t know how high schools are in other parts of the world, but in the US, these are huge buildings with a bunch of hiding places (not that we know). That’s why teachers who aren’t teaching for a certain period have to roam the halls to find students outside of class.
To differentiate between those going to the bathroom and those ditching class, the student has to present a hall pass. In my school, hall passes were good for only five minutes, and the teacher had to write the time on the pass. If a student was caught with an expired hall pass, it’s detention for them. Now that we think of it, it sounds pretty extreme.