I don't think the teacher being punched by a parent is a good analogy. Here are two possible other scenarios that differ from the original in a small way: 1. The teacher sees one student punch another student. 2. Two parents are fighting (this does happen). The teacher does not know who started it.

Regarding judges, we consider it necessary for them to pass judgment but they can gain greater respect sometimes by practicing "judicial minimalism", or saying as little as possible while resolving the specific dispute.