If Spike ever wanted to know where he came from, why doesn't he just ask Celestia or Luna? If they dunno the answer, they can at least put him in the right path! There's his starting point! Question is, what's taking him so long?
Or he doesn't even know they might know? All Twilight said was that he was given to her as an egg, she never said that he came from the school, maybe he doesn't even know?
Even if he DOES know, he's friends with some dragons now! So the Dragon History thing shouldn't stop him now!