Miss Lucy is teaching a poetry lesson to her English class and the topic of conversation falls on soldiers held in World War 2 prison camps.
"One of the boys asked if the fences around the camps had been electrified, and then someone else had said how strange it must have been, living in a place like that, where you could commit suicide any time you liked just by touching a fence."
The class pretends to joke about touching imaginary fences and getting electrocuted, and that's when Miss Lucy remarks about "terrible accidents." She juxtaposes the image of a prisoner of war intentionally killing himself on a fence with both the images of children playfully mocking this act and children accidentally meeting the same fate, and nobody hears her except for Kathy, who identifies this moment as when she first suspects Miss Lucy was trying to tell them the truth about their existence. Miss Lucy makes her think of the students at Hailsham in the same context as prisoners of war, and Kathy starts connecting ideas.
Following this passage Kathy recounts the time Miss Lucy outright tells them who they are and what their purpose is. She contemplates how she and her fellow students have been "told and not told" the truth until this point. Their time at Hailsham has depended on a balance of knowing just enough to keep them complacent in their roles as donors. If they had known the truth all along, would there be a need for electric fencing? Would students be so overcome by the knowledge of their fate that they would choose to decide that fate for themselves and have "accidents"?
For further thought on this idea, we can look to The Myth of Sisyphus by Albert Camus. What Camus says in a nutshell is that philosophers only have one real problem to contemplate: suicide. He claims that life is absurd, and when we are faced with the absurdity, we can choose to end life or recover in some way. And what's more absurd than knowing your life has no purpose other than to end in order for another person to live?