One of six is pretty damn good, and that is before you even consider other factors (such as, how many of those six had a final in one of those buildings at that time.)
That was a lucky guess on Harvard/Police's part. If his local ISP suspected the admin of the blog was on their network (why would they?) then Tor access might help narrow down.
Just like if you're the only person in a country using Tor, writing about stuff internal to that country, yeah, they might notice.
In the Harvard case, as far as we know, they went to everyone running Tor and this kid freaked out immediately and it was case closed. If he has insisted on his rights and not talked about it, or provided another plausible reason (assuming he didn't leave evidence on his computer), they'd have had no solid leads. Or if he had used another network that wasn't Harvard's.