This summer I saw something very similar to the chart you see here. A principal was explaining how she gathered and analyzed student data, and how that data drove her administrative decisions.
You can see, if you click on the chart, that Teacher B didn't do so hot. Because a teacher's reputation precedes him, what's going to happen if parents find out about Teacher B's scores? Will they request a class change? Will they complain? Test scores are scary for teachers because they don't tell the whole story, but they tell an important part of the story.