Well, doesn't seem like all is as was said:

From Twitter:

Question to Karl: @KarlPfeiffer Those old movies are the best though… old clothes and fashion…lmao! When do we get to see you investigate more?

His Response: @LishaMosure On GHI? Never? I haven't been asked to film anything since March. On youtube? I try to keep you up to date, lol

Her Followup: @KarlPfeiffer Well, I am bummed! I keep looking for you on GHI! Boo! Tell them we want u on GHI! U put stuff on Youtube? Didn't know, link?

His Response: @LishaMosure youtube.com/karlwpfeiffer. I'm sure they know I'd love to come back. Wish they'd at least give me a reason, you know?

He had originally talked about going back to school as the reason for doing the one episode and disappearing.  Apparently such may not be the case, which is nothing unusual, proving that that which we see and hear on these programs is not always what happens.

I'm not sure I'd be too quick to side with Pilgrim (or Karl, either, for that matter).  What we have is a situation that has some unanswered questions in it (very common for this franchise).  We believe that GHI films in chunks, ie a bunch of places in England then move on and a bunch of places in South America and then shows them to make them look like they are hopping all around (England to Peru to England to Nicaragua to Germany etc).  Even if Karl had decided to go back to school, what about the other places they probably investigated at about the same time as the one he was in? 

Economically you don't go to England, back to America and take a few days off, back to England and film more, back home for a while, back to England, back home, down to South America.  Britt's twitter confirms this that it is more long hauls.

He (Karl) hasn't heard from them (Pilgrim) since March.  That would be in the middle of the semester, he could still have filmed (especially a few in one area), and then gone back to school, he would not start classes in the middle of a semester.

We also know that not everything we hear or see is fact. What of all the other cadets who were to return for more training…NONE of them reappeared.  Pilgrim is calling more of the shots.  Perhaps Karl wasn't what they were looking for…we don't know that for sure, it's simply conjecture, just like everything else.

Was Karl and going back to school just the excuse that was used to get him out of the way and now he's just talking more and shedding more light?  Bottom line is it's up to Pilgrim who they are going to cast and they can get rid of anyone if they wish. I just don't know if "going back to school" is the valid excuse in this case given timing and MO for filming that I mention above.  I suspect he simply did not work out or perhaps not fit in with the rest of the dynamic and he was let go.  School MAY have been the cover story, it allowed him to get out.