In the novel "Never Let Me Go" we learn about a group of children who are being raised in order to become "donors." What this means is that their purpose in life is to donate their vital organs to others, while they are still alive (sometime after graduating from school).
As terrible as this sounds, when the news are revealed, the students are not truly shocked. This is what Tommy implies in these lines. Even though the authorities at Halisham (the school the children attend) have never fully told the children what their destiny is, somehow they have said enough so that the children always knew at some level what their purpose was. I think this is an example of brainwashing. The teachers have, in various ways, implanted a thought into the mind of the children. However, this was done in a way that was almost impossible to perceive. Therefore, the children are never given a chance to fully examine the idea and oppose it if necessary.