1)  Is Building 12 based on/influenced by "Here is Greenwood" or "PCU"

I am actually quite a Greenwood fan, and I can't deny a certain influence of Greenwood in some aspects of this work, but I believe the directions I'm taking Building 12 in will far remove it from most Greenwood similarities.  As for PCU, at the time that I am writing this FAQ, I have not yet seen PCU, so I cannot speak for an possible similarities.

2)  Is Sarah based on Third-Child.

No, the early scripts, including the introduction of Sarah, were written a full year before I met Third-Child.

Yes, many of the other characters are based at least in part on people I went to college with, though most have been so completely modified that they scarcely resemble the people upon whom they were based. Other characters have no basis on anyone.

3)  Is Atica based on a real college?

Sort of...  The look of it is.  Most of the rest of it is not, though obviously my own experiences from college are bound to creep in to some degree.  Which college?  Well, the one I went to.  If you want anything more specific, I suggest a bit of detective work. ;)