I'd like to take Ulrich's anecdote and staple it to the forehead of several teachers I know. Take this situation:

It is a science class. For the most part, the class is not particularly high performing, but there is small group of high achievers and one child in particular who is dead keen. He has the bit between his teeth and he wants to know this stuff. Science is wonderful, exciting, mindblowing, cool. Especially this stuff they did today. Did you see the way....? He wants to be a scientist. There can be no better job in the world. Imagine doing work like this all the time. And getting paid for it. He works through the set task in record time and storms eagerly up to the teacher. More. Feed me. Input. The teacher is busy working with kids who are struggling to grasp the concept in order to complete the task. He waves a hand in the direction of his desk, where there are some extension tasks for the fast finisher. The trouble is, they are just variations on the theme. The kid has already got this, he wants to move on, he wants to know more. This is like being punished for being quick on the uptake. His pace of work decreases as he works his way through the repetitive material. He is one of the smart kids - he knows he's being fobbed off. He gets bored and starts horsing around. I wonder if I could flick this elastic band out through the open classroom door from here. I wonder if my girlfriend is thinking about me right now. I wonder if I could send her a text message without being noticed.

Going, going, gone. And the kid's report will say that his attitude is "causing concern".