This is not as "far out" as some may think. The Daleks have invaded Earth before, and indeed, they have even visited Coal Hill School before, in "Remembrance of the Daleks," a 7th-Doctor story. They have, for that matter, met Ian Chesterton before, as well. With their new "Dalek sleeper agents," there would be great value in both investigating matters on Earth around this very active area, and even more than that, the Daleks would do well to get a sleeper agent aboard the TARDIS. Based on the statement, "Keep your friends close, and your enemies closer," the value of this is obvious... but who would be watching whom? Having encountered the sleeper agents, he may have figured a way to detect them. Perhaps he would turn this to his advantage... or perhaps the Daleks would turn this to theirs. Regardless, depending on planning within the story-telling process, this could remain undisclosed for episodes or even seasons, as long as this was plotted out well enough. We might not know until Danny's last episode as a companion, assuming The Doctor threw him out after that. Perhaps instead Danny could be "cured" or at least reprogrammed to act primarily from his human side. Again, all this really depends on the writers themselves, assuming they toyed around with this option.
Michael Marcus is a game designer, author, and mad scientist living in Hamtramck, Michigan; his current project list include a series of comic short-stories collectively called "One-Punch," a book on hypnosis and language called "The Prometheus Codex," a collaborative game project called "Art War," and a fun spy story called "The Adventures of Jack Uzi" at http://tinyurl.com/JackUziChannel (for those interested).
