sometimes schools have rules but they don’t care if specific individuals break the rules as long as they do it quietly. This is because the rule can still have the effect they want it to have even if a few specific individuals break it quietly.
I don’t know if this is the case in your example, ZD, but I can see it possibly being so, at least to some extent.
Sometimes, just the fact that the students are forced to do so quietely gives the rule the intended effect.
For example, in my high school, we weren’t allowed to wear knee-his. Most girls did wear knee-his but made sure they sat tzniusly so no one would know.
One time the teacher went around asking everyone if they were wearing knee-his or stockings, and she kicked out everyone who was wearing knee-his. There were approximately 4 girls left in the room. That was the only time that anyone said anything and the teacher even explained afterwards that the only reason she said anything was because she saw a girl not sitting tzniusly.