Both Remembrance of the Daleks and Day of the Doctor were intended as anniversary episodes so they call back to the first episode. If the second episode had the school in it, it never would have reappeared. I don't think being nostalgic counts as introducing an element. When it appears in a random episode that has no reason to feature it, then it'll be a reoccurring element. But the sole purpose of using that exact location is to say "hey look, that's the school that was in An Unearthly Child."
ETA: Speaking of Remembrance, does the Dalek Civil War count as a reoccurring element? I can't remember which episode it appeared in, though.