The loudest teacher on my memory happened for the opposite reason. It was a history teacher and he was asking us to name various Greek gods. That time children here knew Greek panteon remarkably well, we successfully named all the major gods and after we named Aphrodite he said that there was yet another Greek god of love. I knew he was speaking about since before I learned to read (I said, Greek panteon was known well here) and was about to answer, but he made a mistake. He hinted that the word in question has a cognate and that since recently this second word can be heard often. Immediately everyone got shy.  Indeed it was soon after the collapse of (prudish) USSR and many new words became suddenly popular. After several minutes of complete success he couldn't belive that further attempts are futile. His anger grew with every second. His scream "EROS!!!" must have shaken the walls 5 mins later, and he wrote it with meter high letters in the chalkboard.