Based on the short story "Let's Play 'Poison'" by Ray Bradbury. The story was first published in Weird Tales (November 1946).

Mr. Howard: You are not human. You are, shall we say, invaders from another dimension. And as such, you are the enemy. And it is my task to reform your uncivilized little minds. You may think I do not understand you. You are wrong. I understand you all too well. I know that children are as far removed from adults, as monkeys are from men. And it is my duty to forge the missing link. That link is, of course, made of iron. It is called discipline. My thankless task is to train you, even though we look at each other from the opposite sides of the bars of the cage. Sometime I believe that you are little monsters, thrust out of Hell because the Devil can no longer cope with you. You are another species entirely. Your motives, your beliefs, your disobediences. But remember, you have no rights and no privileges and no choice.

Changes from the short story:

* In the episode, the relationship between Michael and Mr. Harmon is more developed, adding more tragedy to Harmon's breakdown and subsequent tough attitude.
* In the episode, Michael dies when he is accidentally hit by a car while chasing after a bully. In the story, the students shove him off a three-story stairwell.
* in the episode, Principal Corliss is introduced as a character, both to bring Harmon back to the school and at the end to wonder where Harmon is. This saves on budget, since it eliminates a speaking role (a passing mother tries to figure out where Harmon is at the end of the story).