At my high school for my first 2 years there was a 2-minute "warning bell" before each class started. When the final bell rang all doors were closed and if you weren't inside a room you were "swept" by hall monitors to detention for the period and missed the class. This was a large sprawling single-floor high school with hallways that went in all directions, not a nice square that you could easily navigate, and it you were unlucky enough to have to go anything close to end-to-end between periods it was a HIKE. They decided to eliminate the warning bell because they felt it was allowing some people to loiter in the hallways until they heard it and still have 2 minutes to get to class, which was true for a small % of people, but what they found when they eliminated the warning bell was the number of missed classes and detentions skyrocketed because students no longer had any idea when the final bell was going to ring. Suddenly good students who never had detention were getting more than one per week.