She's been scratching for nearly a minute. She raises her head and looks at her hand. Three angry red marks appear among the freckles on her skin. A voice in her head tells her to stop, not to do it anymore. So, for the moment, she doesn't scratch. The red marks disappear, but the area still stings.
The teacher asks the class to grade their quizzes, hand them in, and begin writing notes, all of which she does without much thought.
But she keeps scratching. Because if she stops, she's afraid she'll stop feeling at all...
Eventually she stops scratching and the stinging goes away. However, she is all too aware of the numbness that is left behind. She begins scratching again, constantly, and when she stops for more than an hour and the stinging won't go away, she knows she has succeeded in her task.