Yep, I was Harvard '07, and I've TA'ed at Princeton as a grad student. It was pretty obvious in both places that students were collaborating pretty often; for t

Yep, I was Harvard '07, and I've TA'ed at Princeton as a grad student. It was pretty obvious in both places that students were collaborating pretty often; for the classes I taught, in general (though not always) students were cautious enough to hover just below the obvious threshold of phrase-for-phrase copying, so I only challenged students on it infrequently.