<p>A couple of points:</p>
<p>I believe that courses taken for credit at the Harvard Extension School would count for credit at (some) other institutions. Just not at Harvard. Stanford, I believe, gives credit for courses taken through EPGY.</p>
<p>Edited to add:
</p>
<p>I also think that some colleges distinguish between courses taken to fulfill high school requirements (even when taken at a much higher level) and those that are in excess of those requirements. Some colleges allow “double-dipping” and others don’t. </p>
<p>The long and the short is that there is no single answer to the set of questions I listed in response to the OP’s query.</p>