That teacher who stayed after the bell wasn’t getting paid for the extra hour. You were the reason.

The hour after the bell is unpaid and entirely optional. Somebody chose to spend it on a person who could not do anything for them in return, which is a fairly unusual thing for an adult to do with their own evening. At the time it probably looked like nothing much. It was not nothing much.

Go back and tell them, even years later, especially years later. Teachers rarely hear how anything turned out. Be specific about what they did and where it led, since a vague thanks is pleasant and a specific one is something they will keep for the rest of their career.