Here is a strategy I have developed to neutralize the demoralizing effects of such an encounter. First, I listen to everything the person has to say, making the appropriate sympathetic noises and facial expressions. Then, at the conclusion of their story, I ask them this question: "And who was the best teacher you ever had?" You can almost see the Rolodex-flip through their file of schoolhouse memories until they finally find at least one teacher they can speak about positively. Using this strategy shifts the feeling tone of the conversation, it neutralizes the negativity, and anyway it's only fair that if through social convention you're forced to listen to a troubling story, you should also get the opportunity to enjoy an uplifting one.
In my long career I have endured many a doleful worst-teacher story, and that is one reason why I wanted to write a book about great teachers. I love to tell stories about remarkable educators. There are so many fascinating and inspiring stories to tell! To read about twelve of the greatest and most moving teacher stories I have personally discovered, look on amazon.com for my book, Chalkboard Champions.