I doubt they would 'run out' of universities/colleges. There are so many of them-- thousands in the US alone, and then there's the rest of the world...

If they wanted to go back to a school they'd been to before, they would just have to wait long enough so that the same professors were no longer around.
I'd assume they would go under different names as well. That whole thing with the lawyer shows that they apparently have no problem with getting fake identification.