The lag
By the time a teacher hands in a resignation letter, the crisis is usually six months old. The school just did not notice the first five. That is not a teacher problem. It is a leadership problem.
What to watch for
Teachers in distress stop initiating. They stop volunteering for trips, stop suggesting changes, stop laughing in the staffroom. Their marking becomes either obsessively perfect or notably late. They take more sick days, often in clusters.
The conversation
A ten-minute, private, unhurried conversation with no agenda beyond "how are you actually doing" prevents more resignations than any retention bonus. Most schools never have it because no one has the time. That is the failure.