In what situation is a porter not held responsible for a bag of paper?

A porter is typically not held responsible for a bag of paper that is ripped open by school children because this scenario involves an external and unpredictable factor beyond the control of the porter. The actions of school children can be considered as an act of interference or a third-party action that directly leads to damage or loss.

In contrast, situations where the porter might be held responsible usually involve negligence or failure to securely handle and transport the items, such as if the bag is damaged due to weather conditions or dropped while being transported. Additionally, an unattended bag would typically imply a lack of supervision and security, which also points to responsibility resting on the porter or the protocol regarding unattended baggage. Hence, the nature of the interference by school children is the main reason why the porter would not be held accountable in this case.